In TNG in season 3, we see the 'upgrade' of the Starfleet uniforms for the main cast (because of the tightness of the other uniforms). However, the rest of the crew (i.e. the non-main characters) still wear the old uniforms with the colored piping on their shoulders. I expect the real reason for this would be budget constraints for redoing all the uniforms, so they just left them in there, but is there an in-universe explanation for this discrepancy?