even if lemmy had a way to do this, doing it for all the users at lemmy.ml would create a large load of federated network traffic between instances, and at the level of a single instance it would cause a lot of database changes (I’ve heard that the sql schema for lemmy isn’t very optimized).

but at the moment it's generating a lot of server load as is, proportional to how many users on .ml instances are subscribed. every time a post is created or a comment added, the server software tries to post updates to the unreachable instances and fails, filling up the error logs.

if an instance migration tool is on the roadmap, now's the best time and use case to re-prioritize and fast track it. any work the lemmy dev's not taking on on this issue would be work for the admins of all the other instances.