To the OP: we had lunch at BOG on the 25th also! And we shared the roast beef sandwich and had two French Onion soups - no drinks or desserts. We used one QS credit.
I am surprised that they wouldn't let you do this substitution. I would have asked for a manager, as this is totally allowed. We wanted water to drink anyway, so got cups of ice water.
The only place I had an issue was in the China pavilion at Epcot - I wanted to sub the drink for an order of egg rolls (which are a snack credit). The person working the register was a total jerk, but I politely asked for a manager, who was a jerk to his employees (you could observe him behind the counter). He finally "caved in" as though I was asking for something special! Rang it up as a Coke but gave us the egg rolls. Not going back there again! The food wasn't worth it, and neither was the attitude!