And to be honest, I don't know the best way to avoid UX problems when a service change domains.
I mean, besides that many users will lost access/do the reset pass process because of that, the experience to fix this as a user is: go to the pass manager, copy the credentials from the old domain, go to the new login page and paste it, save it as in the new domain, and delete the old domain saved credentials, because that will be saved there forever with no propose and confusing users with duplicated login information.
I guess more advanced password managers maybe notify about domain changes? But at least Firefox and I suppose Chrome, which is massive, don't have that.