Replacing door handles on internal doors is a very straightforward job for a carpenter, joiner or handyman. Handles can usually just be switched like for like, although new holes may need to be drilled for the plates that house the handles if they are a different design from your current ones. If this is the case then your tradesman may also have to fill any screw holes that are exposed and paint the door to create a good finish.
The complete cost of the job will depend largely on the type of handles that you use, with standard DIY superstore models coming in at a lot less than handmade, period or brass handles.
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