The problem is, if they don't have access to their original .ml domain, their accounts are still tied to it. That means if they try to interact, such as subscribing to a community, when the data for that action tries to be sent back (such as updates) it'll go to the .ml domain, which they wouldn't receive.
Lemmy doesn't have a built in way to just change the domain name, or really any of the ActivityPub services AFAIK. You'd have to either really do some hacky stuff to get around it (which could result in unknown issues down the line) or reset everything.