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.)
In other words, it would be clearly illegal for an employer to say simply, "Here's the new employment contract identical to existing but with a new, more aggressive IP ownership clause; sign it or you're fired." Contracts signed under these circumstances have been struck down in court.
However, it would be legal for an employer to say, "Anyone who signs the updated employment contract (with aggressive IP changes) gets free lunches and can work from home 2 days a week." I don't understand the caselaw enough to know what would happen if they provided the incentive and added "or you're fired."
In general, I think the idea is that you have a lot of freedom with new employee agreements, but you cannot just change out an employment agreement on existing employees to strip them of rights.
*IANAL, and it may vary by state.
I would consider new council. I'm fairly certain that continued employment is enough "incentive" to make the contract legally binding. If you're an at-will employee, they can fire you at any moment, so any new contract seems as enforceable as the first.
Tell them you'll check with your lawyer. It's always considered a basic right in developed nations to ask lawyers questions and it usually takes unpredictable amounts of time and it implies that someone with connections is employed to be on your side of any disagreement. That should be the last you hear about it. There's very seldom any need actually to hire a lawyer after saying it.
It's insurance for bad management decisions, effectively. It's useful for all the reasons insurance is useful.
