A couple of questions first—
What do you mean the through-hull is for waste discharge? Is it a bilge pump through-hull or a black water tank pumpout through-hull?
Where on the boat is it located—the transom, the hull topsides?
As for drilling the hole... filling it and then drilling the new hole is one way. Another is to drill a hole through a piece of 3/8" plywood and then attach the plywood to the hull and use it as a "guide" for the hole saw.
Running the holesaw in reverse will allow you to cut the gelcoat without chipping it much.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.