It's inconsistent, but go back in to edit an already created costume at one of the vendors. Every time I've seen this, it's been at Sol or K7. My main character, and also on Bridge Officers. On the final page of editing--for the uniform itself--I will be unable to select certain options, usually later on in the order of items. There's not really a consistency or pattern to it. What may not work the first time may work the second or fifth or tenth, and it may be a different uniform piece.
I can, basically, select a different uniform option, but as soon as I move the mouse off it to 'lock' it reverts to the previous option.