That’s been asked before, but I don’t remember ever seeing an answer. If it’s in the Wiki, I don’t see it. You’re talking about moving the hosting, but leaving the registration as is, right?
My guess is that support would have to do it if you didn’t want downtime.
You could back it up, delete it, then re-add it to the other account – but sometimes, the records don’t flush right out, so you might have a little downtime. If I did it that way, I would instantly send support a ticket asking them to flush the domain from the old account, just in case it takes awhile.
I’ve removed domains and INSTANTLY added them back… but I also had one that was still stuck in there months after removing it… which is why I’d send the ticket.
Maybe there’s some trick with users/groups that allows a slicker way of doing it… but I don’t know. Maybe someone that has actually needed to do it before will chime in.
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