@developing_agent Nono. What I mean is that if you have a local account, and then federated auth is implemented, any interaction from your home base account in its capacity, with a server you already have a local account in, would end up with a duplicate account situation that's hard to fix (one local, one remote), while what you want is to convert the existing local account to remote.
My gut feeling is the safest solution to this is email matching. If you're already requiring emails for account creation, then they can be used to check for a preexisting local account. You'd have to verify via email confirmation, but it would avoid the risk, and it can also be used for remote account recovery when you lose your home server. The flip side is it exposes your email to all servers, and in fact allows "does this email exist in this server" lookups by alll third parties (modulo rate limits etc), but I feel the advantages outweigh the disadvantages here (though you could still turn the feature off if you know/accept the risks, there's room for more paranoid opt in choices here).
There are subtle tunables here, e.g. you could do a bit of "proof of work" stuff for email lookups to avoid blatant spammer/scraper abuse.
