What is the best way to transfer an out of state custody order and also ask for modification?

My current parenting plan was issued out of the state of WA. However, my ex and my child moved to Boston, MA approx 18 months ago. I am also no longer a resident of WA. My child is 14 and never wanted to move. I would like to file for a modification as my child at minimum wants to see me more and have a lot more contact with me and at maximum wants to move back home. It is my understanding that, given how long my child has resided in MA, MA has jurisdiction. My ex's attorney, who is in WA, insists WA continues to have jurisdiction. Not a single person in the parenting plan resides in WA any longer. In addition, my child has indicated some disturbing circumstances at my ex's residence and I would like to ask for a guardian ad litem. What is the process for transferring the parenting plan while also wanting to modify it and ask for a GAL?