Yes, Roman religion underwent significant changes pre-Christianity. Fair warning, there have been and still remain to be written entire books on this subject, so I’ll do my best to highlight some important moments of change but this is by no means exhaustive, and the more I add the more I realize I’m leaving out. In the interests of ending somewhere, there are two moments in particular that might be helpful to focus on (although it is important to remember that these were hardly the only times of change to Roman religion): the period of the Punic Wars and the Augustan period. Both are significant inflection points in Roman culture anyway, and between them we can see the inclusion of new cults, the development of new divinities, religious reorganization led by the imperial government, and of course the implementation of the imperial cult, to give just a few places where changes appeared.

The adoption of foreign cults in Rome is a fairly well-attested process, and the Republican period provides a number of examples of this. Through syncretism, foreign gods found themselves with Roman identities and local divinities were translated and integrated. In addition to this process of integrating new gods into the existing pantheon, Romans were also willing to adopt entirely new cults. In the third century BCE alone, Rome saw the arrival of cults to Asclepios, Venus Erycina, and Magna Mater (Cybele), and the implementation of new games and festivals. While this is a trend that is in no way isolated to the third century, the cult to Magna Mater shows one way foreign cults found mainstream footing at Rome. Magna Mater was brought from Phrygia to Rome via Ostia in 205 BCE and her baetyl was housed in the Temple of Victory on the Palatine. Livy reports that she was brought to Rome because an oracle found in the Sibylline Books connecting her arrival with victory over a foreign enemy in Italy (Livy 29.10), giving her arrival a root in Rome’s own pantheon and legitimizing a cult to a foreign goddess. By 204 the Ludi Megalenses, sometimes also referred to as the Megalesia or the Megalensia, were instituted and in 194 became one of the four ludi scaenici that included scripted performances of Roman comedy. The Ludi Megalenses of 191 BCE, where Plautus’ Pseudolus was staged, coincided with the dedication of the temple for Magna Mater, giving Cybele her own temple in Rome. With Roman expansion the cult spread into the provinces as a part of the Roman pantheon. To address your question about coincidence with moments of turmoil, Magna Mater is also an interesting case. Livy’s account implies that the Romans turned to the assistance of a foreign goddess in a moment of military need against the Carthaginians. However, Gruen points out that by 205 BCE Rome was no longer on the brink of defeat when Magna Mater was brought to Rome, and argues instead that the adoption of Magna Mater was part of a larger effort to gain more influence in the Greek sphere. Orlin (2010) makes a similar argument regarding Venus Erycina, reading her adoption in Rome as a way to strengthen ties with Punic Sicily. While Cybele appears in Greece as early as the 5th century BCE and did have connections there, another possible reason for her arrival in the 3rd century was her role in the Trojan foundation myth for Rome, which was gaining traction during that period. Venus Erycina, brought to Rome only twelve years earlier in 217 BCE, had an explicit connection to Aeneas’ journey from Troy to Rome, and Magna Mater, with her home on Mt. Ida, became integrated into myths about Rome’s founding in later periods. If she did have this connection with the Trojan cycle in the earlier period, the timing of bringing Magna Mater to Rome just after Venus Erycina could be a way of strengthening the emerging national identity for the Romans as successors to Aeneas and the Trojans. Both goddesses were brought to Rome at a moment when Rome's emerging mythos of its place in the larger Mediterranean world put the city's history into contact with the cultures these goddesses represented.

Throughout the Republican period we also see the repeated emergence of a group of deities Clark identifies as “divine qualities”, like Fides and Virtus, who also figure into larger patterns of heterogeneity in Roman religious practice. The third century BCE saw the dedication of a number of new temples to personified deities, including but not limited to Victoria, Spes, and Fides. Orlin (2010) argues, following Fears, that the adoption of a cult to Victory at this time was, like the adoption of cults to Venus Erycina and Magna Mater, meant to bring the Greek world into the Roman sphere as allies as part of a larger “theology of Victory” established at that time, while Feeney posits that the simultaneous establishment of cults to Mens and Venus Erycina are a reaction to military losses to Carthage. However, since we cannot establish clear origins for most of these cults, although plenty have tried, it’s not the most helpful to think in terms of the Roman adoption of Greek cults as though they have simply been subsumed into Roman culture one for one nor in terms of aspects of cults being wholly Roman. Clark decisively refutes the tendency of Orlin’s 1997 book to frame cults in terms of Greek and Roman elements as though Greek and Roman identity were monolithic, unchanging things, which could just as easily apply to Orlin’s arguments about the primarily political adoption of Venus Erycina in Rome. Clark’s book, which I highly recommend if you’re interested, delves into how the diverse groups interacting with the Roman cults to divine qualities produced cults which were themselves changing and variable (for instance the establishment of a rival shrine to Pudicitia Plebeia when certain women were not allowed into the shrine to Pudicitia Patricia).

Moving forward to the Augustan period, I want to highlight two major changes that are, in essence, quite political in nature. The first is of course the introduction of the imperial cult. While this cult certainly did not spring fully formed from nothing — the goddess Roma had her own cult as far back as the second century BCE, for instance — the cult surrounding the emperor and the imperial family was, of course, specific to the imperial period. The imperial cult, or, following Friesen, the imperial cults, changed and adapted to the existing religious practices of the different local provinces where it appeared, making it less uniform than its state ties might suggest.

In this period there is also a reorganization, or “reimagining”, that Flower calls “one of the most striking and wide-reaching aspects of the princeps’ religious program” of the cult of the lares at Roman crossroads. Augustus gave these local deities a new epithet, renaming the lares compitales the lares augusti, restored a temple to the lares, and possibly revived their local festival. Flower argues that the reimagining of local lares compitales, who were above all local cults tied to the neighborhoods (and the political workings of those neighborhoods) where they were located, as lares augusti gave the emperor presence and visibility within the existing network of local neighborhoods and was particularly notable as one of the few Augustan religious changes not put forward as a restoration or revival of an existing practice.

Roman religion was definitely an evolving, multifarious sort of thing, incorporating new cults and easily reimagining and reworking old ones, a process driven in large part by the diversity of the different groups who participated in religious practice both spatially and temporally. Hopefully this has been helpful in narrowing in on a few specific instances where we can see these changes happening.

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Clark, Anna. Divine Qualities: Cult and Community in Republican Rome. Oxford: Oxford University Press, 2007.

Fears, J. Rufus. “The Theology of Victory at Rome. Approaches and Problems.” ANRW 2.17.2 (1981): 827-948.

Feeney, Denis. Literature and Religion at Rome: Cultures, Contexts, and Beliefs. Cambridge: Cambridge University Press, 1998.

Flower, Harriet I. The Dancing Lares and the Serpent in the Garden: Religion at the Roman Street Corner. Princeton: Princeton University Press, 2017.

Franko, George Fredric. “Festivals, Producers, Theatrical Spaces, and Records.” In The Oxford Handbook of Greek and Roman Comedy, edited by Michael Fontaine and Adele C. Scafuro. Oxford: Oxford University Press, 2014.

Friesen, Steven J. “Normal Religion, or, Words Fail Us: A Response to Karl Galinsky’s ‘The Cult of the Roman Emperor: Uniter or Divider?’” In Rome and Religion: A Cross-Disciplinary Dialogue on the Imperial Cult. Edited by Jeffrey Brodd and Jonathan L. Reed. Atlanta: Society of Biblical Literature, 2011.

Gruen, Erich S. Studies in Greek Culture and Roman Policy. Leiden: Brill, 1990.

Orlin, Erich. Foreign Cults in Rome: Creating a Roman Empire. Oxford: Oxford University Press, 2010.

_________. Temples, Religion and Politics in the Roman Republic. Leiden: Brill, 1997.