If they were provisioned to a DNS name, you can change the DNS record to reflect the new IP (and you'll need to change the IP in the AP system profile if you were using one).
If they were provisioned to the old IP and it is no longer available, you'll need to reprovision them. If you still have it available by chance during a cutover period, you can create a provisioning profile with the new controller IP/DNS name. So long as the RAPs can still get to the old IP, they'll grab the new provisioning profile and update themselves for the new ISPs connection. But unfortunately if they have already cut you off, you'll need to reprovision them.