If you're dead set on having an underhand grip on the front hand, then make sure your new front hand is overhand. This is backwards to how most people do it, but it is the way this guy describes it:
http://www.stormchasers.org.za/tips/mikefickgybe.htmlIn other words, your two front hands (old and new) have to be opposite grip to each other. This stops your elbows knocking together, and is THE tip for improving your rig flips.
To convince yourself, next time you're rigged up at the beach grab the sail as if you're sailing. Now start to flip the sail, and STOP when your new front hand grabs the boom. So you'll have one hand each side of the mast, and your arms will be crossed. Stay that way for a minute or so.
Try this with both hands undergrip, both hands overgrip, and then one each way.
I promise you'll be a convert