garage door opener motor running constantly
At 20 years the controller probably has failed. But if he did not look at it I would question his diagnosis. There are things in some openers like belts that break, some direct drive couplers the coupler breaks or a sheer pin,  if the door did not make its turn off cog or sensor the motor would keep running trying to make it. The frequency band has changed over the years along with the safety eyes and even motor requirements have changed for energy efficiency so it is possible the newer model controller is not compatible but I would want to check the mechanical out. When you push the button can you hear the motor stop and reverse directions? If so I would say it is a broken drive component not the controller.