The sentence in all the mss. (except the Codex Coislin. II., which has ὁ τραπεὶς) begins οὐ τραπείς.  The Ben. Ed. propose simply to substitute εἰ for οὐ, and render “Si enim conversus est, proprium constituit corpus, quod videlicet densata in ipsa deitate, substitit.”  I have endeavoured to force a possible meaning on the Greek as it stands, though παχυνθείσης more naturally refers to the unorthodox change than to the orthodox conjunction.  The original is οὐ γὰρ τραπεὶς οἰκεῖον ὑπεστήσατο σῶμα, ὅπερ, παχυνθείσης αὐτῷ τῆς θεϊκῆς φύσεως, ὑπέστη.