Two persons reach an understanding before getting married and planning to live together. Some of these understanding are explicit and some of them are implicit in nature. During marriage ceremony some of them take vow within the framework of a particular religion. However, if one of the partner wants to change his religion then s/he has to take the other partner into confidence and attempt to convince him/her about his/her decision. I believe some degree of enlightenment is required on the part of both the partners to overcome such challenges. After all the marriage is between two individuals and not between different religions. So, yes one can change his/her religion after marriage within the framework of mutual understanding.
Once someone gets married, it is their choice to keep or switch to a different religion. They should talk this over with their spouse before the marriage to ensure that there are no problems. If their spouse cannot accept them for who they are, they definitely should not be married.
I believe that it is best to stay true to yourself. You should live your life for you and not for anyone else. Do not let the fear of being judged to stop you from being yourself. If you change, deep down you still will believe what you believe. You are only lying.