What would happen if?

I bought this Romisen RC-29 light which has an XP-E

I think its this one
http://dx.com/p/romisen-rc-29-cree-q5-wc-3-mode-180-lumen-convex-lens-led-flashlight-with-strap-1-aa-1-14500-50679

And replaced the LED with with an XP-G2 and no driver swap?

At 700mA the XP-E uses about 3.3 volts and an XP-G uses 3.05 volts

Would it fry the led or soak up enough current to burn the driver or something else?

It will be fine. Throw might be reduced. Total output should be higher.

Yep, it would be fine.