[GeeTwo]

If you only replace one pair, replace the one with less weight (away from the center of gravity). For most robots this year with a convex rectangular base, this would be the rear wheels. We used this configuration for Ultimate Ascent, and we had the two solid wheels in the front and two (idle) omni wheels in the rear. Our CoG was quite near the center of volume. We did have a bit of fishtailing at high speed going forward, but at low to moderate speed, we were fine; I don't imagine you'll be doing much sprinting in Recycle Rush. Interestingly, we had less fishtailing in reverse. This was presumably because more of the weight was over our solid driven wheels.

A better solution is to use six solid 6" wheels as the chassis was designed to use. The center wheels are dropped by 1/8", so the wheelbase is only one half or the other for purposes of "tank mode" turns, but full length for purposes of not falling over forwards or backwards.