To be more clear, one take's a rotary carbide file chucked into some kind of drill or Dremel and from OUTSIDE the boat, one grinds away the inner most edge of the thruhull's hole. This will allow the flange outside the thruhull to be removed. If one takes one's time, there will be no damage to the fiberglass, just a new thruhull needs to be purchased.
Frozen seacocks can be soaked in an auto parts cleaning
machine for easier disassembly.
I think most seacocks are frozen from lack of usage.
I cleaned Skylark's seacocks on a bench grinder with a soft wire brush and they looked totally new. I will get another 40 years use from them.