Have you checked whether the larger 8" wheels will collide with the front and back "C" frames ? If so, is it possible to notch the frame to accommodate the extra 1" wheel radius ? Else you have to move the axles and then the belts don't fit....
Or extend the frame
Also keep in mind when going from 6 to 8 inch and not changing any gear or belt pulley ratios, the robot will go slightly less than 8/6 times faster. Though your driver does has a "throttle", it might be somewhat harder to control the robot in close in maneuvering. And the robot will have 8/6 less torque (in a pushing match).