The running joke (as told by one of the TNG writers at a convention) was that the E-D happened to be in transition at the time of "Generations" and the crew were being issued new uniforms automatically the next time they took a shower. This is borne out when Geordi came back from being kidnapped, bathed, and he was in the new uniform. The logic sorta stops though when you realize neither Troi nor Crusher change for the whole movie. Worf stayed in his uniform the whole time, but hey, he's Klingon.
It was also painfully obvious the rest of the cast were just using the available uniforms from DS9 (except for Stewart and Spiner, who had custom ones made for them). The extras in the background could be chosen to fit them, but not the principals; as a result, Riker's sleeves were way too short and had to be rolled up to disguise the fact, and LaForge's sleeves were way too long!