There will be a 1/32" gap around the through hull. That is not all that large, a good quality caulk could fill it.
The "right way" to fill a hole is to grind out a 12:1 bevel and then apply layers of glass and fair it. I don't think it is necessary to go that route to fill this small a hole. I'd be inclined to clean the edges of the existing hole to make sure all the caulking is removed and sand the hole with 80 grit. Then fill the hole with thickened epoxy and drill the new hole. This isn't a structural repair and the new through hull will seal the hold, you're just trying to fill in a 1/32nd inch gap around the new through hull.
While it sounds easy to just enlarge the hole to take the next larger diameter through hull, it really isn't that easy. I tried enlarging a hole this year and ended up filling the hole and then redialing it. If there is a reason to go with a larger drain, then go ahead and fill the hole and drill a new one. Either way it is the same job.