If they want to ensure that current owners are tempted and willing to upgrade to the new watch, they'll include a trade in program. I think however we'll not see one between version 1 and 2 initially (or at all). If the adoption rate of a version 2 apple watch is low, then perhaps they'll adopt it down the road, sort of like their price cut, or when the apple watch 3 rolls off the assembly line.
If Apple can avoid giving away money (which in a sense a trade in program is), they will and the apple watch is such a new beast, its hard to say what existing owners will or won't do