A while back, the startup I worked at was acquired by a large company known for its aggressive legal department. Everyone had to sign an agreement, including both intellectual property and non-compete clauses, or be fired. One guy actually did refuse, and was fired. A few years later, I left that big company to work for another startup. I was concerned about that non-compete, so I did a bit of checking and found that it was NOT A CONTRACT as far as the law was concerned. You see, a contract by definition requires consideration on both sides. The relevant courts had repeatedly found that an offer of initial employment counted as consideration, but an offer of continued employment did not. Thus, what I had signed was not a contract. Legally, it wasn't a non-binding or unenforceable contract. It was a non-entity. A piece of paper with some irrelevant squiggles on it.
I've heard that other ex-employees (and their lawyers and the courts) reaching the same conclusion is the reason that the big company in question adopted a policy of technically firing everyone from any startup they acquired, and then re-hiring them immediately under the same terms. The sole effect was to turn that continued employment into initial employment, non-consideration into consideration, validating the rest of the contract. It's now standard practice throughout the industry.