I bought a Storm Parallax Effect ball that is like new but has been drilled.  I want to try a better ball than my plastic beginner ball.  The span is about 1/4" too short but everything else looks okay.  The pin is up and to the right  a couple inches for a right handed bowler.
So, to increase the span can I just have the thumb hole plugged and redrilled and perhaps replace the finger  grips?

If you go to a shop that knows what they’re doing, that should be something they could do for you.

I used to do it all the time. Worked for me.

 As Juggernaut said.....YES! I do it all the time as well. Just make sure they measure your span correctly before they do it.

So, quick question to the drilling gurus:  Would it be cheaper for him to have the thumb slugged and redrilled or plugged and redrilled?

Depending on the shop, it should be close to the same price.

 I’ve had it done both ways and, if they’re good with color matching, I actually prefer plugging.

 If it is just a slight adjustment, slugging isn’t too bad, but you have to be close enough to drill completely through both slugs, as U.S.B.C. rules state that no voids can be left in the ball. Slugs leave a small void if not drilled completely through.