I really think if you explained everything to her, that she wouldnt mind or be as hurt. Are you having bridal showers and parties and such? just explain to her that it would be too much responsibility for her to do it all on her own, and it would be best if she was your honorary bridesmaid.. make it sound better, or let her keep her title and expalin to your friend that she is going to be in your life forever.. and it would mean a lot if she would take on the roles of being moh but be okay with not standing first in line??!!
If this was my situation, I would keep her as my moh, my best friend wouldnt mind stepping in but not having the big title, she knows where she stands in my heart and in my life, she's an adult and gets it.
I wish you luck and I hope it all works out for you!!