It is of course currently possible to abandon a fortress, and later come back to the same site with different dwarves. Consider the reverse: I'd like to be able to abandon a fortress, and subsequently take the same group of dwarves (perhaps with some items brought from the old fortress) to a new site.
If done in peacetime, it could probably be assumed that all dwarves make it to the new site, and start there with a wagon containing all the items they want from the old fortress (perhaps using something like the "Bring goods to depot" menu to choose which items are brought along). On the other hand, if the fortress is abandoned during a siege, only the dwarves that make it to the edge of the map alive will be at the new site, and only with whatever items they can carry with them.
Inspired, of course, by the backstory of the dwarves of the Lonely Mountain and the dragon Smaug from The Hobbit. The front gate of a fortress is attacked by a megabeast; many dwarves are killed but some escape through another exit. Some settle at another site; others roam around the world map in adventure mode and eventually return to the original site to drive out the dragon and reclaim the fortress. This particular example would require individual characters being transferrable between Fortress Mode and Adventure mode, but simply settling at a new site, building up a larger military, and then returning to the original site in force would work just as well. I would love to see this sort of thing be possible in DF.