I've been counseled that an employer cannot compel you to sign a new contract after starting a job, unless there is an incentive to offset any rights/benefits being forfeited. There's a huge gray area in terms of incentive/benefit value, but it's clearly illegal if there are no advantages offered by the new contract and it only benefits the employer.

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.