When a crusader gets polymorphed, all crusaders sharing their slot on the bench are polymorphed too. The Bush Whacker, Nate Dragon, their alternates, and slot 21-23 crusaders are never polymorphed. To other crusaders' formation abilities, crusaders retain their tags while frogs (for example, Emo Werewolf's Lone Wolf ability still treats polymorphed humans as humans).
Based on your current area, the crusaders that get polymorphed are as follows:
|51—100, 451—500||101—150, 501—550|