From the evidence we've seen, the TOS uniform color doesn't refer to personal qualifications for command a vessel, but to which division the person is currently assigned. More likely is that shipboard organization was changed so that communications fell under the Engineering and Ship's Services division rather than the Command division.
You may be right. But it's never been made clear exactly how those assignments come about. The captain and first officer put together duty rosters and crew assignments. I'd guess each pair does things somewhat differently. Some captains make assignments based on experience and past performance: others may do so based on each crew member's stated goals, provided that said goals aren't unrealistic and don't create logjams at certain duty stations.
Moreover, is a person's duty uniform based only on his or her assignment, or does personal preference also factor into the decision? Spock could have worn command gold as first officer, but outside of WNMHGB, he was always seen wearing science blue. It was only in TWOK and afterwards that he wore the colors of the command division, having achieved his own captaincy by then. Sulu wore command colors during the five-year mission before switching to gold in TWOK and back to command white in TUC after getting
Excelsior.