For me, the problem seemed to be that I didn't have enough credits to cover the cost of the costume change, even though the cost was clearly less than the amount of credits I had.
Let me explain: I had a costume change that cost 110 EC, and I had 130 EC. After making my changes in the Costume Editor, the cost correctly showed as 110 EC. I pressed Modify, and nothing happened; I remained in my old uniform.
However, after I sold some stuff and got up to 1000 EC, the Costume change went through without a hitch. And only 110 EC was subtracted, even though I needed a greater amount before the change would go through.
My guess is that, under the hood, the Costume transaction code tests against a higher EC value than what's displayed, even though it ends up subtracting the right amount in the end. Definitely a subtle bug, if it is one.