Can I move before modifying child custody?
If you can relocate and still follow the child custody schedule as spelled out in your separation agreement, then you can go ahead and move.
If relocating will make the child custody schedule difficult or impossible to follow, then you should seek a change to the custody terms before you relocate. Otherwise, if you were to relocate and then miss a scheduled drop-off with the other parent for example, you would be breaching the separation agreement. Your ex could file a breach of contract lawsuit against you for not following the terms.