Futaba 127's have to stay in their original LOW or HIGH band. But you can make them any frequency within that band,

With the high cost of service to day, it is not cost effective to send a receiver of a plain four channel transmitter in for service. You don't by used ones at the swap meets either.

Throw them away and buy new if they give the slightest bit of trouble. Your plane/engine is worth more than a receiver.