My daughters, age 18 & 15 have lived with grandfather in Michigan for 10 yrs. I live in Georgia. My 15 yr old wants to return with me due to an abusive situation with her and my step-mother. My father has full custody which I GAVE to him when I was in an abusive situation and was a drug addict. DEFACS was not involved in them going to live with him. It was MY decision. I have since been clean over 3 years and have a good stable life. Is she old enough to decide on her own? Does it have to go to courts again if both parties agree it's best for her right now? What would need to be done to get this to happen?

A child can not decide where they live. They can testify in court as to what they would like to happen, but you will still have to file a motion in court for a change in custody, you can present your evidence and the other side can fight it if they wish.

... Does it have to go to courts again if both parties agree it's best for her right now?
...
No, but your father could always change his mind.