I've never had much luck with this, pure cloud accounts SIP address will always match the UPN. Hybrid accounts don't have to. Typically, I'll communicate with the users and change it to the UPN before moving up, that way I never have to think about it again. If I've already moved it up, and need to change it, I've usually given up and moved it back down, edited it, ran Azure AD Connect sync, then moved it back up. If you script it out, it doesn't take too much time and you can do it in bulk. I imagine there's a better way, but I haven't found it.
