A lock replacement means swapping the whole lock on a door, with its case, its mechanism and its cylinder, for a new one. It should not be confused with a cylinder replacement, which only swaps the cylinder.
It is needed when the case is broken or worn, when it is a lever lock and you want to move to another system, or when you want more security with a multipoint lock. In many cases changing the cylinder is enough and the lock is still fine.
The new one has to fit the space left by the old one or the door will have to be altered. Before deciding, send us a photo of the edge with the door open. More under lock replacement and cylinder replacement.