So transitions between pantheons are fairly common in the polytheistic religions; the Olympians replaced the Titans, the Aesir replaced the Vanir, Marduk replaced Ea, and so on. My question is, does this mean that the Titans and the Vanir, among others, were probably worshipped in their own right at some point in the distant past, and that the victory of their successors in myth represented a historical shift to a new pantheon?
Briefly, no, and in some cases the so-called "older" generations appear to have been historically later additions to the pantheons. An interesting example is Helios, mythologically the older sun god before the upstart Apollo showed up, but there is very little evidence of independent cult to Helios in early Greece. By the Hellenistic period, however, he's a lot more popular. Dionysus, conversely, has a mythology that portrays him as being a newcomer, younger than other Olympians, and post-dating the establishment of the major Greek poleis. Archaeologically, Dionysus is one of the oldest gods in the Greek pantheon, maybe even inherited from the Minoans.