If done properly it won't ruin the integrity of the shoe. If the sole is new the stitching is not disturbed at all. If the sole has been worn to far -or- in your case it sounds like it was damaged at the toe. A new leather tip is attached and stitched on. Then, the flush mounted toe plate is added.
Welcome to the forum and good luck in your career.Try a tongue pad. It will push your foot back into the heel of your shoe.If you have a competent shoe repair shop in your area they can add a back liner which will take up some room in the heel. Even if the heel lining wears out a new back liner will solve that problem.