Now that migrants are pulled from historical figures, it's easy to get migrants from old forts to move to your new ones.
My second fortress in the new version (first died to those new gas fronts pretty early on) died after I mismanaged a magma pumpstack and I decided to abandon.
In my third fortress, I started getting migrants from the old fort (same names, same skills, same relationships with the other migrants that they had back in Fort #2). Here's what I found:
1. Dwarves are "exported" from the old fort in the EXACT same mental condition as when they left. It freaked me out at first when I got a few unhappy migrants who claimed to have seen the death of their friends. Then I realized they were being tormented by the memories of their old home. This isn't really of benefit to anyone unless you abandon your fort at the height of ecstatic dwarves, and that way your migrants are happy, or you really want a lot of near-tantrum migrants for some reason...
2. Old Military Habits Die hard. When a pair of my military dwarves from Fort #2 arrived, they were "active", and considered themselves a part of their old squad. This stopped when I assigned them to a new squad, and I didn't really do much research on it.
3. Injuries are exported in a funny way. One of my Axedwarves lost a leg to a Ruther in the old fort. In this new fort, he's still missing that leg, but can walk just fine, whereas before he needed a crutch to walk around. So he essentially lost his "lost ability to stand" tag in the trade over. On the other hand, my hammerdwarf who lost his arm still lost his ability to grasp, so something fishy is going on there in terms of what is kept and what is not.
4. What they were wearing does not seem to transfer over. My Military Dwarves didn't have any of their old armor or weapons, and arrived in the same generic migrant outfits (one of everything of random leathers and clothes).
5. What they were hauling MIGHT transfer over (this is a really big one!!!). A number of my migrants arrived carrying marble pots of Dwarven Wine that I recognized as coming from the #2 Fort. I don't know if the pots got migrated over because the migrants were hauling them when they were exported, or if they were carrying them when they were exported, or if they were drinking from them when they were exported. I haven't seen any other hauling items get exported, so I'm wondering what makes the booze they were carrying special enough to get a data packet to get migrated over with.
6. I haven't determined a pattern of which Dwarves are more likely to migrate over (older or newer dwarves, dwarves with more history...) (and I don't remember how close this site is to Fort #2, so I don't know if that contributes).
7. I haven't gotten a Dwarf who produced an artifact to show up as a migrant, so I don't know if you can get those kind of dwarves to produce second artifacts in their new homes....
EDIT: 8. My old Chief Medical Dwarf arrived with a new pet (some kind of bird I think), and I don't recognize any of the other pets as being owned by the dwarves in the #2 Fort, which suggests to me that pets are NOT drawn from history, but are instead randomly generated at migration, but I haven't investigated this in full yet.