What if I need to replace an old Skin Fitting that has been epoxied into the hull ?

If for example a 1" toilet outlet needed to be replaced with a 1½" toilet outlet;
Wedge a wooden bung into the Skin Fitting from the outside and use a hole saw to drill out the fitting with a diameter 1mm larger than the diameter of the threaded part of the new 1½" Skin Fitting. Chisel off the surround from the outside of the hull and fit the new 1½" Skin Fitting.