You can see an illustration of them here
. The Generations action figures
also featured the crew wearing these uniforms (most likely due to the long lead time when it comes to merchandise).
I've heard a couple of reasons for why these weren't used. As the caption on the second photo says, Rick Berman and Co. were concerned about introducing too many new elements at once. On the other hand, Ron Moore mentions on the DVD commentary that costume designer Robert Blackman was concerned about how the series uniforms would hold up on the big screen.