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.