Radar, I've had similar experiences. 'Guarded' dressing rooms are interesting, because they force a person to make gender calls on perfect strangers. Long before I ever came out to myself, I was getting directed to the mens' side of dressing rooms. Usually I just went with it, but the most awkward time was when I was with my mother. She always insisted I try everything on, and so I followed her (dragging my feet) into the womens' side of the dressing room. The lady 'guarding' held me back in shock and said indignantly "Young man! Your dressing room is over *there*!" I was maybe 14 and thought it was kinda cool, especially since I had long hair back then; my mother was mortified. I've noticed that if you happen to be trying on all mens' clothes and the dressing 'guard' saw you shopping in the mens' dept., they're more apt to direct you to the mens' dressing room.
And yeah, people definitely aren't used to gender-changing. If they're used to calling you a 'female,' they aren't going to change that no matter what happens (unless you tell them & they're cool with it, of course). I'm not out where I work, and my voice has often been mistaken for that of another guy who works there. However, they aren't going to put two and two together unless I tell them; until that point, I remain unquestionably 'she.' Once you start passing regularly with total strangers and the store clerks & whatnot who knew you from before are still ma'aming you, it may be time to find new stores (if that's possible)... Of course, you could hope that they don't recognize you after you transition, but if you go there very frequently they will probably have time to adjust to your shifting visage and will still be able to recognize you (though they might notice something's up).
