How do I replace drive belt for ProForm 530i treadmill?

I have a ProForm 530i treadmill that had stopped working and the Treadmill Doctor helped diagnose that it was the motor controller board, not the motor as I had suspected. I replaced the motor controller (from Treadmill Doctor) and also ordered a treadmill kit that included belt cleaner and lubricant for the walking belt and belt dressing and conditioner for the drive belt. They suggested keeping the walking belt lubricated to keep wear on the motor down and use the belt dressing to prevent the drive belt from slipping. I have had good success with this for the past year (the treadmill is 5 years old now), but now the drive belt is slipping and the belt dressing isn't helping. I cannot get any more tension on the belt (the motor is positioned as far back as it can go). Is it possible that the belt has worn or stretched so that it must be replaced? If I replace the belt, I see how to loosen the motor to remove the front of the belt, but how do you remove the roller to slip the old belt off and the new one on? If the belt dowsn't need replacing, what is causing the slippage?