A server can have per-user salts, which may be migrated together with the hash.

They better have per-user salt. Otherwise that would defeat the entire purpose of salt, which is to make sure that it isn't obvious if two users have the same password.

What’s the issue with taking ownership of your posts with you?

I think the question is will they allow it. Though I'm sure there's weird edge cases. I'm not an expert on the Ferdiverse, but let's say you hope from Lemmy.world to Threads or something like that. But Lemmy.ca banned threads, now your posts have moved. It kind of creates a weird migration.

Though the counter argument also exists "Why do you need to take ownership of posts?" I don't know if that's as valuable to go through the hassle. If anything moderator positions is what would matter more and can be done through a process rather than migration. (promote B, remove A)

But like I said, I'm not an expert so I'm still learning.

B. Just migrate the salt, too. A server can have per-user salts, which may be migrated together with the hash.

If ever server has per-user salts, great. But that's again a question for implementation. You're making a huge assumption that's how it works, and there's no server dependent values. I mean even the hashing algorithm should be able to be proprietary be different between servers (At least that's how I'd do it).

C. If you already have control over someone else’s account, what additional benefit does migrating serve?

Because control can be reverted, or limited. I mean if I can step to Bieber's laptop when he's in the bathroom, that's not the same as having his login in and password... If lemmy.world sees someone has broken in and locks the account, or Bieber changes his password, your access is lost, where as if you migrate it, can lemmy.world revoke the migration? And if that's the case, doesn't that create some strange problems?

I'm not saying this is a bad idea, but I think you have to think more about the edge cases. Though this does have me now thinking about what's the value of this migration?