Where does Mom live?
Where does Dad live?
And since at one point, because the grandparents HAD custody, they somehow fit into this picture, where do they live?
Jurisidiction will follow the child, ONLY if NO ONE lives in the original jurisdiction. IF this is true, take the order to your new county and ask the clerk how to file it. They may or may not be allowed to answer that question -- they can't practice law, so when you ask a HOW question, if the clerk's office had determined that's practicing law, they won't answer. If it's determined that it's simple, a basic no brainer, they will help. Each county in each state in this country will give their own answer.
OTHERWISE, the party who still lives there can object.....and sometimes that works, and sometimes that doesn't.
There are folks here who have seen it go either way.