Before we had the Season One update drop, my lead character had two uniform slots -- her regular one, and a second one for being a member of an in-game Fleet.
Well, the new season came out, and she got a third slot -- off-duty. Fantastic. And we got the new costume options! Great!
So, I went into my original unfiorm slot and made some changes (I gave her Ponytail 6 and changed her Borg implant to one of the in-eye ones surrounded by one of the new brackets, instead of leaving the full block around her head).
Then, for the heck of it, I made her an off-duty uniform. The choices were a bit sparse but it was pretty slick.
Finally, I went into my second uniform-slot -- the one for my fleet uniform -- and just changed the hair and implant options so that her appearance would match her primary uniform appearance.
I hit save... and was told "this slot is not unlocked."
So. Long story short, the costume slot I got for being in a fleet no longer allows me to change its appearance. I can still switch into that uniform, but I can't customize it.