I think once the FC Uniforms were introduced, they decided Class B would become the more formal look than Class A. Definitely by the time Ross and Daugherty were wearing them.The Lower Decks uniforms and the 2380s uniforms from Picard could be a sort of Class A and Class B sort of affair, like the TNG uniforms and the DS9/Voyager ones. Problem there is that Lower Decks is supposed to be the more laid back setting and they're wearing the more formal looking uniform.
And I'm very much looking forward to Lower Decks!