Can Rak Tulkhesh turn into a PC with his Change Shape?

Real talk.
Can Rak Tulkhesh turn into a PC with his Change Shape?
Polymorph substitutes CR for level if there’s no CR, would this?
Can my BBEG turn into my party’s wizard and hit her with her own fireball? Have you considered the possibility that your party’s wizard may have secretly been Rak Tulkhesh from the very beginning?

— Keith Baker (@HellcowKeith) November 22, 2019