Same issue exists regarding old holes in the door faces.
Replacing an old mortise door lock with a modern lock.
Pull the lock out of the edge of the door.
If you want to switch to a cylindrical design typical modern knobset mechanism is housed in the knobs and a cross bore rather than in a mortise or a rim surface mount lock you ll either want to leave the old mortise lock in place or glue in a block cut to exactly fill the space it occupied since otherwise the empty space would weaken the door.
Once you ve removed the door lock template you have a space perfectly primed for your new lock.
The problem with replacing the mortise with a bored cylindrical lock is that the new lock is smaller than the old and really has to be in the same location on the door as it s the strongest part of the door.
Instead of fixing the mortise lock it can be replaced with another mortise lock.
You d have to fill the mortise hole with a solid block of wood which might well entail digging out more of the door to get a good fit.
Use a screwdriver to remove all screws rather than a drill.
For budget conscious people such as landlords not willing to lavish their tenants with expensive locks another choice is retrofitting the door with conventional tubular locks such as those available at big box stores.
Remove the screws attaching the inside face plate to the edge of the door as well as the plates on the door faces.
Remove the face plates prying them up with the screwdriver if needed.
Place two screws from your new door lock kit in the doorjamb to hold the template in place then use the bit from your lock installation kit to drill a hole where the new doorknob will go.
Cleaning and repairing an antique mortise door lock.