They are very simple to replace, I took mine out to replace the outboard plastic bearing fitted within the roller. I was able to do it without removing the unit or backing off the roller to tyre gap. [With different rollers you might however need to do this gap readjustment].
All you need to do is significantly slacken off the bolts that attach the motor/gearbox unit to the fabricated Reich body assembly. [two off 10 mm spanners]. So the motor unit can move well away from the body.
Pushing the roller towards the motor you then can get enough clearance for the roller to come off the outer bearing spigot. Once the roller is off this spigot move the roller to the side a bit and draw it out towards you. Its motor end is slotted to take the drive from a pin that is through the gearbox output shaft. If that pin is in a bad state then it will need replacing. Mine was not but my brothers was.
If you replace with some after market rollers I understand you need to fit a different intermediate span plastic support bearing because the rollers are smaller diameter, but I have not been there.
Useful tip I noted to help me next time:
Initially take out one upper mounting bolt and replace it temporarily with a longer one. That way you can get more clearance between the gearbox and Reich body, with the weight of the motor hanging on that bolt. That bolt can also help retain alignment for when doing up the bolts after replacing roller.