We just boarded Discovery Princess today and tried to swap out the 8 little bottles of liquor for beer and were told no also.  We could change them for something called “mixed soft drinks” but not specifically Diet Coke which is all I’d be tempted by.    I don’t remember seeing Princess ever state in writing the minibar could be swapped but we’ve been doing it for years now and wish we’d known before boarding.  I guess we must be in the minority by not purchasing the plus package but we don’t