Recently we were contracted to change the locks on a caravan. The new owners were concerned about how many keys existed and who had them so changing the locks was the best solution.
The main entry/exit door was of aluminium construction with a Mortice Sash Lock fitted. We had to cut the handles off as they were welded on with rust. Once they were removed the old lock came out relatively easy and the new lock slotted in the hole nicely. Securing it was awkward as the holes for he fixing screws were too large and fitting larger screws meant that the face plate was not flush, however, we enlarged the lock holes and used the larger screws.
Once the new handles were replaced, the lock was very secure indeed.