The original TOS uniforms were great for 1960's television, but were too garish and dated-looking by the time TMP was made--they would have to been modified with darker colors and overall remade in a more flattering contemporary style like they were in Star Trek XI.
But I think the uniforms really needed to change in TMP to signify that some time had passed since Kirk's 5-year mission, IMO.
But I think the uniforms really needed to change in TMP to signify that some time had passed since Kirk's 5-year mission, IMO.