Earlier quoted context omitted.
This is what startups pay a lawyer to do. It's not that hard: restructure the company ownership in a long document with complicated terms. Get the employee to sign it and voila, early employees are screwed. The only real response is just not signing, but they will tell you the company will die if you don't do it, and you probably will just take it rather than fighting. At least I did.
But signing something without not understanding it is too naive. So, assume I already left the company and exercised a big bag of common options and so I have a decent amount of common shares. If they tell me to sign something that looks shady I just say: "No, it looks shady". What then, can they do?
It doesn't take a lot of imagination to see that this can happen. People are generally naive, and there are just a few that will resist and fight.