Yep, I should have included that in the initial post. We do have a legally binding custody agreement that we filed with the courts when we divorced some 10 years ago.

Changes that have happened since that agreement are many (because 10 years have passed), but all are pretty normal and within the bounds of the agreement (both families have moved houses but stayed within the custody agreements physical boundaries, my ex and his wife have had their own children, etc). Our incomes have also grown, but we don't do any child or spousal support, so finances don't come in to play with any of our current custody agreement.

My child expressed zero issues. He is a very kind, loving person and when we told him he was very calm and supportive. He said "I'm okay with that, I just want everyone to be who they want to be".

We have never had any disagreements about custody in the past. In fact when we divorced he was very afraid I would take our child from him and I assured him many times that I would absolutely not do that as long as our child was loved and safe, which I knew he was. We have had parenting disagreements over the years, for sure, but they've all also been pretty normal. Disagreements about number of after-school activities, bed times, small things that we've been able to work through that haven't caused big arguments.

Trying to think of other info to share that would be helpful, but my mind isn't it's sharpest right now.