Swapping a single roller on a door that's down and unplugged isn't beyond a patient homeowner with the right tools - you release the retaining bracket, slide the old roller stem out, and seat the new one. Where it goes wrong is the top roller on either side, which sits inside the track near the bend into the horizontal rail, under spring tension even with the opener disconnected. Pulling that bracket loose with the door in the wrong position can let the section drop or the track spring away from the wall. If you're only dealing with the bottom two rollers and you're comfortable with a door that's genuinely at rest, it's a reasonable DIY job. Anything involving the top rollers on a torsion system, we'd rather you call.