Depends on the reason the user could not login. If they really just forgot, then maybe that would nice UX... maybe. It could also be concerning to an unaware us

Depends on the reason the user could not login. If they really just forgot, then maybe that would nice UX... maybe. It could also be concerning to an unaware user that that somehow the site knew my old password (even though _we_ know they are just comparing hashes). If the login was disabled due to the site knowing the hash of the old password appeared in a rainbow table somewhere, it would be a terrible idea to just let them use it again.