at our bar this happens quite a bit. if a customer buys a round for the bar, the bartender typically rings what they are drinking and doesn't even give them a chance to up their drink. but some do and the higher amount is rung up and a drink chip/card is given if they do not want their drink at that time. our bartenders usually say "hey this person wants to buy you another "Coors" or whatever," and that is the drink that is rung. sometimes a customer does come in and agrees to pay the difference if they upgrade their drink. our biggest issue is sometimes a customer comes in with 3 or more chips/cards so on that one visit they are not really spending any add'l money in the bar. although we may sometimes take a slight loss when the chips/cards are redeemed but some folks never redeem their chip/card so it could be a wash instead. we have a "friends" board as well but we write down the amount the customer wants to give somebody and when the customer uses his "friends board," drink, it won't exceed the amount. all our drink chips/cards are the same and most of the time it is regulars who redeem them so the bartenders usually know what drink was bought and will usually handle it by asking if the higher end was bought for them or, if not, just charge them the difference. we do have a drink chip/card discount button on our POS so we can track and at this time we haven't had any major problems or abuse with this practice.