Disabled, planning to move from grandmothers to mothers?

Okay, here is my situation. I am 17 years old and have muscular dystrophy. I am in a wheelchair, but still have a large range of movement and am of sound mind. My 18th birthday is steadily approaching and I've been waiting to move in with my mother, who is completely willing to take care of me. The only reason I live with my grandma is because as a child my mother could not financially support me. Now that I'm older, she is ready to do this. My mother still has custody over me; my grandma only has a notebook paper signed by my mom stating that she has permission to take me to the doctors, and it's not even laminated. Should I take any precaution in moving? Am I just paranoid? Either way, I'm still afraid that my grandma will try to pull something.