Pretty much stated. They redesigned the uniforms for First Contact and to maintain continuity they had to switch the DS9 crew over. Which was funny since you'd see the extras in the old uniforms for a number of episodes until they had enough made for them. Heck, the Admirals didn't get new uniforms until season 6.
As for Voyager, they kept the old uniforms since they were separated from the Federation. Though by they were aware of the uniform change by Message in a Bottle and definitely by Pathfinder.