DPHSbasbllplyer,
Adjusting your grip on the ball, even after going to the full set position, is not a balk. However, you have to be aware of the fact that some umpires pull the trigger (call a balk) on movement faster than others.
If, in fact, you didn't move anything other than your hand while adjusting your grip then a balk should not have been called. But when you choose to adjust after you're fully set that's the risk you take. Once the ump calls the balk your coach can argue the point but it's very unlikely he's going to un-call it.