He retains command of his own ship, but to mark the transition from single-ship commodore to multi-ship commodore, he shifts from the starship badge (or whatever) to the Starbase/Starfleet Command badge. Maybe he's required to, or maybe he just has the option.
That's a fresh rationalization attempt which I really do like a lot. Assuming all hell breaks loose (as it did in "The Ultimate Computer") and there'd be multiple visual reports from the various starship captains and confusion just a quick glance at Wesley would abundantly make it clear that he is in charge.
I was wondering about the absence of the command symbol in Decker's insignia myself. Had there been one I would have never assumed in the first place that the Wesley's "flower" could be the insignia of the Lexington