The toga continued to be worn in Late Antiquity, and was most associated with the senatorial order and certain offices, such as the consulship. The chlamys costume had military origins, and was adopted over the course of the third century A.D. (becoming the attire of the elite classes throughout the period) as part of a gradual shift in clothing styles.

Through the stationing of the army along the frontiers, local and non-Roman recruitment, and other regional factors, Roman military attire gradually changed over the course of the Principate. This is unsurprising, as the army was a continuously evolving institution, forever appropriating aspects of the cultures with which it interacted. As early as the late second century A.D., and most certainly by the third, long-sleeved tunics and tight-fitting trousers became common among soldiers along the Rhine and Danube frontiers. Long-sleeved tunics and more baggy, equestrian trousers were part of the Iranian cultural tradition, and were spread westwards by soldiers stationed in Mesopotamia. As these types of attire were worn (in some form) by both the provincial societies from which the army recruited and by the foreign communities it fought (and from which the army also recruited), it is unsurprising that military attire changed during the late second and third centuries A.D., even though it is unclear which region (Germanic west or Danubian/Iranian east) had a greater affect.

The chlamys, a sleeveless, semi-circular cloak of ankle-length, attached at the right shoulder by a broach, had it’s origins as a military cloak, entering service sometime during this period (the late second-early third centuries). The instability of the third century ended the senatorial monopoly on the imperial throne, as men of more modest provincial backgrounds were elevated (and assassinated) by the soldiers. Military dress thus found it’s way into civilian fashions, a process further facilitated by emperors retaining sizable escort armies. While Diocletian and Constantine, through their bureaucratic and military reforms, reshaped the imperial elite, military traditions continued to shape the character of Late Antique bureaucratic service (such that bureaucratic service was referred to as militia, or military service, even if it was nothing of the sort). It is in this period, the early- to mid-fourth century, that the chlamys costume was adopted as the official dress of officialdom.

This costume consisted of footed-hose trousers, open-faced leather shoes, and a knee-length, long-sleeved tunic. The hem, neck, shoulders, and cuffs of the tunic were decorated with embroidered appliques, the color and design of which reflected the wearer’s status and rank. The addition of these appliques is one of the defining elements of Late Antique military and civilian dress. At the waist was worn a leather belt with reinforced metal fittings (cingulum). The belt was a central element of military wear, and thus became one of the most important bits of insignia for Late Antique civilian officials (such that “to put on/put aside the belt” was synonymous with entering or retiring from bureaucratic service). Over this was worn the chlamys. It’s vertical edges were decorated with a pair of rectangular embroidered panels (tablia), the color of which differed from that of the cloak. The chlamys costume was thus colorful and highly-decorated, and conveyed instantly the wearer’s rank and importance (much like a modern dress uniform does today). This was something of importance in the extremely hierarchical world of Late Roman officialdom.

The chlamys costume was, by the fourth century, the official dress of the imperial court, the administration, and the officer corps. By contrast, the toga became increasingly identified with the senatorial order. From the fourth through the seventh centuries A.D. (evidence for it’s use later is patchy at best), the toga became a purely ceremonial garment. It was no longer worn daily, as it had been by elites during earlier periods. This is reflected by changes in how it was draped, as the toga now had folded bands that extended across the breast and between the legs. Throughout Late Antiquity, emperors continued to insist on the wearing of this now-ceremonial toga by the senatorial class. On formal occasions in Constantinople, or when sitting in judgment, senators were required, as per an edict of 382, to wear the toga, and forbidden to wear the “awe-inspiring military cloak” (the chlamys). This may have been to elevate the prestige of the senatorial class, which was now greatly expanded (due to the creation of a Constantinopolitan senate, and the conveying of senatorial status after holding certain senior posts) and not particularly influential. It may also have been to demonstrate the status of “New Rome” (Constantinople). Late Antique togas were no longer plain white, but, like the chlamys could be decorated to convey status within the hierarchy of the senatorial class. The most decorated of these was the consular toga, which consisted of a purple toga with gold embroidery.

To finally answer your question, the toga was replaced by the chlamys costume as the official attire of the imperial elite during the late third and fourth centuries, as military fashions exerted a greater influence over civilian dress. The toga continued to be worn by the senatorial elite, where it became the official dress. Both the toga and chlamys, in their own unique way, conveyed at a glance the rank and status of the wearer. Thus, both garments reflected the culture of display that characterized Late Roman elite attire (and non elite, seen by the highly-decorated equipment and clothing of the army), through which individuals demonstrated status through highly-decorated clothing and accouterments.

References:

Maria G. Parani. “Defining Personal Space: Dress and Accessories in Late Antiquity” in Objects in Use: Material Spatiality in Late Antiquity (Late Antique Archaeology 5- 2007), ed. L. Lavan, E. Swift, and T. Putzeys (Leiden and Boston), 497-529.

J.N.C. Coulston. “Late Roman Military Equipment Culture” in War and Warfare in Late Antiquity: Current Perspectives, (2013) ed. Alexander Sarantis and Neil Christie (Leiden and Boston), 463-492.

Christopher Kelly. Ruling the Later Roman Empire. First Harvard University Press, 2004.