@developing_agent You'd just have to migrate the account to have non local scope (presumably set a scope/domain field). Whether this breaks past mentions depends on whether those are stored by name or by ID... (I hope id)
But the more interesting thing is designing a flow to "take over" a local account and migrate to remote identity without risk of ending up with two unmergeable accounts too easily...
As for multiple clients, an existing legacy client miiiight be able to keep its auth token and stay logged in but there are a lot of asterisks on that. And to log back in you'd need, at minimum, "app passwords" or something.