If it is NOT a micro guide, I'd switch the tip out to a larger diameter one. Most of your last guide eyes (before the tip) on a "non Micro" are somewhere around 4 to 8mm. The average size tiptop (ID) is 6mm.

If it DOES have micro guide eyes, I would change it out with a Fly rod tip top slightly larger than the one it currently has. Fly rod tops will get down with a smaller ID.

Changing the top top diameter isn't going to have that much (if any) felt effect on the rod or it's action. Many ways to skin a cat from changing your connecting knot, shortening it up, changing your line, to changing the top. You have options.