Someone recently asked me what I thought of using the word addendum in connection with contracts. I’m not crazy about it.
Black’s Law Dictionary defines addendum as “Something to be added, esp. to a document; a supplement.” So an addendum adds something to a contract, but it’s not clear whether you’re amending or supplementing the contract. (MSCD 17.10 discusses the distinction between amending and supplementing.)
Furthermore, addendum is a Latinism, and it isn’t one of those Latinisms that’s so ingrained that they can now be accepted as English.
So I suggest that you use instead amendment or supplement, whichever applies.