A warranty is not an End User Licensing Agreement (EULA). An EULA is used when the "seller" (licensor) retains ownership of the product (typically, a copyright or a patented process) and is allowing the licensee to use that property under specific conditions. The licensing agreement sets out the terms under which the "buyer" (licensee) can use the owner's property (like an apartment rental lease).
There are arguments made that the typical, "click here to use this software/service" EULA should be unenforceable, because the end user typically hasn't given "informed consent," they've just clicked the button in order to get along with their life. But even if some terms of the EULA may be unenforceable, since copyrights and trademarks are involved, whatever laws exist would still apply (no, you can't sell copies just because the rest of the EULA is unenforceable).
On the other hand, a warranty is a one-way statement, a promise from the seller to the buyer that the seller will continue to be responsible for what they sold, under specific conditions. The law requires the seller to keep that promise, because that promise helped persuade the buyer to buy.
Within the bounds set by law, the seller can promise as little or as much as they wish. But like any other promise, the recipient of the promise can't turn around six months later and tell the person who made the promise, "I also want you to give me stuff that you didn't promise when I bought it." (Well, they can try, but the seller is under no obligation to agree.)
So, to answer more directly, I'd have to say, "No." If I make you a promise, you can't change what I promised.