The best advice I can give you is wait to hear what they say first, but start to plan ahead. For all you know it may not be something bad as I know I've gone to meetings that had nothing to do with me before. Just regular corporate stuff. But if it is, ask yourself if you would!d sacrifice this job for your transition? And would you sacrifice your transition for your job? It sukcs to be put in that position, but consider where you are right now and how you feel. Personally, I would probably come out to hr if they told me to start adopting a stereotypical male appearance as j think that may change their position, but its really your call based on how you feel and your knowledge of the situation.
I'm sorry you are going through this, though I hope it goes well and or turns out to be nothing bad.