At the beginning of the Thieves Guild quest line I didn't trade in my one armor piece for the better armor piece. Now Tonilia won't give me the option to trade in my armor. I have all the Thieves Guild armor but she only gives me the option to sell stuff to her or ask what other services the Guild provides.
According to Tonilia's page the the dialog will disappear if you don't use it (although when it says it disappears makes no sense), if you still have her fencing service and the quests are still progressing all your missing out on is one item with a stronger enchantment .If you really want it i think you'll have to reload a save when the dialog was still there.
Another way to get out of it (if not being able to trade in the armor is stopping your progress) is reload and kill Gulum-Ei as upgrading the armor is the reward for him living (but you'll lose him as a fence of cause).