Koldik Steelskin is totally right. Something interesting I thought about is also how casting Polymorph onto another character would impact that character if they were concentrating on a different spell. It wouldn't be able to protect the other character from making their concentration checks for the spell they were holding, so sadly it wouldn't be able to do that for a wizard that cast it on themselves.
It can be a bit of a bummer, so I might play a little fast and lose with it at times. The thought of a wizard casting it, becoming a trex and getting popped out due to a failed concentration check before being able to do anything as a trex wouldn't be a ton of fun for them.