It seems like there's a few issues with body swapping that make the previous unit unaccessible. However, more research has to be made.
unretire-anyone a night troll, you can travel and rampage as it but if you attempt to retire you will give in to starvation, even if attempting to do so at your own lair.
body swapping to a peasant, retiring, and unretiring in a site actually retires the night creature properly at that site.
playing as an outsider with no group or site links, body swapping to someone else, leaving site and coming back, I believe it's impossible to find the wandering adventurer during that game session.
However, if you retire, the wandering adventurer will be unretirable and they will be at an "unknown location". Picking that person will randomly choose a valid playable site to wander them to.
bodyswap needs to either create a wandering army controller for wanderers, a lair site to treat as their home, or force retire them that emulates vanilla behaviour, besides forced starvation stuff cuz that's lame
It seems like there's a few issues with body swapping that make the previous unit unaccessible. However, more research has to be made.