Dominion: Can travellers (Peasant, Page) be Upgraded or Trade Routed?
Yes. The cards in the traveler lines have costs so that they can be considered for cards such as Upgrade - in all such cases, you just ignore the "*" in their cost (just as you do for Peddler). So it is perfectly legitimate to trash a Disciple with Upgrade in order to gain a Gold.

The main thing that you can't do with those cards is interact with their piles outside of the exchange mechanism - you can't return them with Ambassador, and you can't gain them with something like Workshop.