Cast Members have never came up to my family and I and said sorry you can not use those mugs anymore. Besides no one but your family member would know when they were purchased if the designs on the mugs never change.
I even asked a few Cast Members about this and they said WDW does not care if guests bring back old mugs, because the syrup and everything else involved with the mugs costs WDW nothing. Besides Coca Cola gives them the Syrup for free, in return WDW advertises Coca Cola all around the parks and etc. Those are words from Cast Members and not my own opinion, so I am just the messenger here. Finally one Cast Member told me they have a mug from Wilderness Lodge, that they have used in the past while at the All Star Resorts.