If you have sectional doors and one of the panels is damaged, you can replace only this one. There's only an extreme possibility that you won't find exactly the same panel if the door is very old. If you have one-piece door and the damage is not repairable, you'll have to replace it.
They could be thicker but this is not always the case. They tend to be thicker with polystyrene insulation materials or when you choose to get multiple panel layers (three or four layers) for higher insulation. According to our experts, polyurethane insulation materials are spread evenly among panels and they don't affect the door's thickness.
Replacement becomes necessary only when a certain part is beyond repair. The notion that a defective opener or a broken hinge cannot be repaired is a myth. All garage door parts are rated for at least five to ten years with manufacturer’s guarantee, so they can be easily repaired with proper inspection and overhaul. The decision of repair or replacement should depend upon the condition of the part.