They will probably tell you that you must sign it, or else terrible consequences will ensue. If those consequences involve the company, brush it off, because that's not your problem. If they involve you, then see if they're actually willing and able to carry them out before you make a decision.
I've heard many stories of attempted unilateral changes in employment terms like this where employees refused to sign. In some cases, they got mildly harassed for a while and then it petered out. In some cases, nobody even followed up. In some cases, they were able to negotiate the language to everybody's satisfaction. I haven't heard of anybody getting fired for it. (I'm sure it has happened, it just doesn't seem all that common.)