Is amending my contract as simple as changing the list of beverages in it?

It might be, but it is usually more complicated than that. Different sizes and types of beverages have different manufacturing costs and may sell faster or slower depending on how popular they are with your students. Those differences can affect the terms of the contract.  For example, you may have received cash advances or non-cash items that need to be considered in your discussion.  Or, under the terms of your existing contract, you may be entitled to future guaranteed payments or non-cash items that may need to be adjusted or reconsidered,  Also, you may have committed sales volumes that you are expected to meet during the length of the contract.  Not all contracts include all of these terms, but if yours does, you should be ready to have a constructive dialog with your vendor(s).