I don't know how to make the call on pre-cutting the old or just gooping over top.
When I make an overflow, it's never a thing of beauty and it usually takes me 2 or 3 tries before it's completely watertight (I usually just goop over top).
I suppose you could try gooping over top and see how it goes. Hope for a lucky break
Or if you're feeling ambitious, just cut it and reseal it. But it's probably worth resealing on both sides of the overflow (if you can).