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Maybe. There has actually been a couple of times where he had/has almost gotten turned into the form accidentally. The first time was when he was younger (he was already in that form, but something triggered that), then when Doctor Fell and White Knight tried to disect him in "Promises, Promises" his nanites were going berserk, which could've turned him into it and lastly, in "Six Minus Six" when he was placed on Caesar's memory machine, he almost turned into it as well. So if I had to guess, I would say that if Rex is put in dire trouble or something (which could most likely happen) he'd turn into his full form.