The best course of action is to watch all transactions very carefully. Most of these servicing transfers happen without incident, so don't be too nervous. The terms of your mortgage will not change.
Potential problems are payments in transit during the time the file is being setup with the new servicer. Also, there can be some confusion with property taxes or homeowners insurance if you have an escrow. So, keep an eye on due dates and check in to make certain your account has made the transition without incident.
Federal rules govern the procedures the banks must follow when they sell your loan. You should expect to receive a "hello" letter from the new bank and a "goodbye" letter from the old bank. Each of these letters must provide a toll free customer service number for your use and most importantly, a very special address for written correspondence.
This very special address is different than a payment address. Federal rules require that any written correspondence you mail to this address be logged and tracked. Use of this address helps the consumer get a prompt response to any inquiry concerning the servicing of their mortgage loan.
It happens to most of us, so you're not alone. ;)