Unless the backing blocks on the interior side are rotten/leaking or heavily corroded I would suggest leaving them alone. Another way to check for corrosion is grinding back the surface of the thru hull on the exterior. If it is bright and shiny with no red spots then it is still sound. The red color indicates loss of lead due to galvanic corrosion.
The reason I suggest leaving them as-is is because the thru-hulls on my N-41 are in excellent condition. When we did the bottom job last year I removed one of the thru-hulls in the stern (the engine raw water intake) and it was in great shape with no signs of corrosion. The only reason I did not put it back in is because I damaged the threads while removing it. The bronze thru-hull I replaced it with was much lighter in construction. The OD of the flange was small and it did not fit perfectly to the molded recess to the hull (there is gelcoat beneath the fitting). So on the forward thru-hulls I ground back the old fairing compund painted the surfaces with epoxy and then re-faired them with thickened epoxy.