[chumbucket]

Overlooked this thread. Sorry. Welcome ;D
You've probably already dealt with this by now. I don't think there's any trick or easy solution other that trying to cut the foam in sections and you can possibly replace it when the new tank is installed. Or, pick up some new two part expandable pour-in foam when completed. I just think it'd make the job go easier by removing and getting everything cleaned up nice before the new tank gets dropped in.