Funny how I never feel supported when people remember my existence only to use it as a Gotcha...
Why would a woman be distressed by my, or some other transman's, presence in her changing areas? Sure, she might have a few seconds of alarm if we happen to look particularly male (which is very much on the rare side), but she would very soon realise that we're female, at which point she wouldn't care anymore, neo-penis or not.
Moreover, we were socialised as women, so we know how to act politely and respectfully in order to put others at ease. Also, we would possibly be the ones most upset by the situation, the ones who least want to be there, the ones who want to get out as quickly as possible, and the woman would pick up on that too.
So yeah, very little cause for alarm all in all.