how many times can you EFFECTIVELY change the drilling pattern(via plugging) on a ball without drastically lowering the effectiveness of the core since you are drilling it out little bit at a time.

my situation is that i was given 15 different bowling balls(2-20yrs old) from a teammate for free(he's moving 5 hours away in summer) and included among them was a storm trauma and a storm XFactor.  This person has a TON more revs than myself and much more ball speed also(cranker style) so these two balls are drilled to go very long....too long(for my liking) for myself at the present time.  i could change the surface but would rather change layouts IF possible.

thanks in advance for any replys
btw the xfactor is on its second drill pattern(plugged once)....and the trauma is on its third pattern(probably will leave the same)