How hard is it to replace a sliding glass patio door with french doors

If the french door frame is the same size as the glass door frame, its not difficult. If the f.d. frame is larger than you can have problems. If the f.d. frame is smaller, then the opening can be reworked to accommadate the smaller f.d. frame.

Patio doors and exterior French doors are both shipped in a box (framing). The window and door manufacturer I work for, makes both types of exterior doors, and as long as they are the same sizing, it's just a total switch out. Here is a link to give you an idea of what the pre installed units look like.

Hope that helps..