So who knows anything about how this stuff works?
I've been playing around with it lately. I made a fort, built it up to a barony, then retired it for a new one which I got up to a duchy. By the time I went back to unretire the original fort to see how it had changed over the years it had been conquered by goblins. Apparently immediately after I founded the new fort. Given the way invasions work it now strikes me that it would probably be quite an accomplishment to be able to retire a fort, go away for a decade or so, and come back to see what level of changes happened.
I can verify that a few things were altered. When I went back to reclaim the ruined retired fortress, a lot of my former dwarves were still there. Unfortunately I reclaimed as a rival dwarf civilization so they were all hostile. I was able to ascertain, before he completely finished caving in the heads of my settlers with a pick, that one of my legendary soldiers had been promoted to Hammerer (a position which, for obvious reasons, I tend to leave unfilled).
Other than that it was just like any reclaimed fort, with stuff strewn around, but I imagine that was because it was ruined. No sign of goblins either, but apparently the dwarf population was not completely wiped out.
When just playing around with it and reclaiming a fort as quickly as I could after retiring it I didn't notice any changes at all.
Does anyone know if the population cap in the init is preserved after the fort is retired?