Assuming that you are replacing your French doors with like-for-like doors then your builder, carpenter or joiner will not have to first expand the wall opening in order for them to fit. You should also be able to use the existing door frame and possibly some of the current fittings.
Your tradesman will remove the old doors and dispose of them for you as you wish. They will then tidy up the area where the new doors are to be fitted, filling any cracks in door frames or strengthening the spot where the hinges will be attached if this is needed. The new doors can then be hung and adjusted as necessary, including cutting for height if needed.
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