Maybe instead of giving everyone a memo "New style uniform from now on", Starfleet told them "Okay, we are phasing the old uniform style out. You are allowed to choose the new or old style at your own discretion for now. All personal must change to the new uniforms by stardate xxxxx.xxxxx."
That works for me. Voyager just happened to have taken off after that change. Or Janeway decided to go on a power trip and order everyone to change early.
I remember hearing that after the proposed new uniforms were 86ed, they had to scramble. For some reason they didn't want the TNG uniforms in the movie or in Voyager. They settled on the DS9 uniforms but didn't have the budget for tailoring new ones for the TNG cast. Anyone who could fit into an existing DS9 uniform wore one for at least part of the movie. I believe they even went out and said:
- Stewart and Frakes were able to fit into Brooks' or off the shelf uniforms
- Spiner was able to fit into Meaney's or an off the shelf uniform.
- Dorn could not fit into Meaney's costume and they had nothing in his size. The same was the case for Sirtis and McFadden.