Maybe the agreement is "we will accelerate vesting of your unvested equity if you sign this new agreement"? If that's the case then it doesn't sound nearly so coercive to me.
In general, an agreement to agree is not an agreement. A requirement for a "general release" to be signed at some time in the future is iffy. And that's before labor law issues.
Someone with a copy of that contract should run it through OpenAI's contract analyzer.