An "in universe" explanation"? Um...
Well, in the late TNG/early DS9 era, it was clear that space station personel wore a different uniform to those on a starship. Starship crews were evidently more 'Starfleet formal', whereas base personel were 'Starfleet casual'. But in Voyager, the ship crew also wears the DS9 'Starfleet casual' look, having evidently been launched after Starfleet had issued the order saying everybody has to switch over to the other uniforms. So, "Generations" probably depicts the changeover from one to the other (though the old uniform doesn't appear to have been completely phased out at the end).
I do think it was a bad idea changing the uniforms (the movies and Voyager too -- I'd have prefered if they'd stayed consistent and kept different uniform types for base and starship personel). IIRC, the whole point of the DS9 uniform was that it was supposed to be more like something you'd wear in that kind of dressed-down enviroment. It's the kind of uniform an engineer would have on when he's got his sleeves up and is tinkering with components. Maybe Geordi's team should have been wearing them on the Enterprise more often?
The thing is that's not really true... we saw starbases wear TNG style uniforms... in fact, in late-TNG/early-DS9 the only time we ever
saw the DS9 style uniform in use was on DS9 itself. There's nothing to suggest that one is for ships and the other for starbases.
I didn't like that it seemed to have no reason or method of why they would change. In the third season of TNG, remember they had a uniform change. And the lower-ranked officers were the ones that wore the 1st and 2nd season uniforms (a jumpsuit with no collar and a stripe along the shoulder). They should've done that in Generations.
It's not as wacky and random as you you might think. Throughout the course of the movie several of the crew change their clothes. From the old style to the new style, it's that simple. Picard is the only one who changes back after he messes up his new uniform fighting Soran on Viridian III.
Having two types of uniforms in service at the same time just doesn't seem that crazy to me I guess.