The upgrades of Peasant and Page (Soldier, Fugitive, Disciple etc) are not in the supply. But can they still be trashed by Trade Route, or can they be upgraded by Upgrade? For example, a Disciple has a technical value of 5 - can it be upgraded to Gold by Upgrade?
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.