The black and grey uniforms were produced for First Contact to replace the
DS9/Voyager versions. It was decided that those w/ the color on top and black on the bottom did not look very good on the big screen in "Generations" (which is why there is actually a mixture of two uniform styles in Generations). In order to mesh
DS9 w/ the movies,
DS9's uniforms had to be changed as well. Which was done a year later since FC was a bit further ahead in the timeline than the
DS9 episodes that were in production when FC was in production. Voyager didn't change b/c they were stuck in the Delta quadrant.
In-universe timeline-
"Rapture" (the first episode of
DS9 to feature the new uniforms) stardate: between 50049.3 and 50416.2
"First Contact" stardate: 50893.5
"Concerning Flight" stardate: 51386.4
"First Contact" was released in '96, "Concerning flight" was released was released in '97
[/b]