A single bent or cracked panel looks like a bolt-off, bolt-on job, and sometimes it is. The trouble starts when the panel is part of a door old enough that current stock doesn't share its dimensions - you can buy a panel that's close and spend a weekend finding out close isn't close enough. Torsion springs under tension are the other reason to stop: they store enough force to cause real injury, and a door built around 1977 has often never had that spring serviced, which means the tension on it is an unknown quantity, not a known one. If the panel is cosmetic and the door still opens and closes evenly, a careful owner can often do the swap. If the damage affects the track alignment or the spring needs to come off first, that's where we'd rather you call before you start.