Well the guy side of a CD does remain unchanged, the issue is that the guy side was only ever half of the personality and has maybe been artificially skewed somewhat to fit in with gender expectations. Elements of the femme side do leak through into the guy mode, but not in sufficient amounts that it becomes obvious what is going on.
The CD is at pains to keep the two sides of his personality separate from each other. It is like holding apart two parts of a magnet. There is a natural force that wants the two parts to join together but the resulting whole is deemed unacceptable by society. Because of the constant exertion to keep the two parts separate, CDs become overly focused and sensitive to the different gender roles and make a big fuss over it.
The guy is still there but most CDs, when dressed, the guy side is placed on the back burner and the femme side comes out to play often in an exaggerated fun-loving manner. Obviously from the outside it can appear that the CD has changed into someone else but it really is the same personality which has been artificially split apart into two camps in order to fit in with societal expectations.
This is not a healthy or wise thing to do. I believe CDs should stop fighting the natural desire to fuse together back into a whole person at least in private environments.
For SOs the big problem after dealing with the shock and unsettling feelings of abnormality, is to understand the duality their partner has had to cultivate to fit in and not to allow their traditional gender conditioning to cloud their perceptions of their "man".