It's a different matter for an asset acquisition but generally, if you're performing the same role, in certain jurisdictions your existing contract terms have to be respected (this is the case in Europe at least, I would assume that in the US, the position is more flexible though).
You'd like to think your new employer is reasonable and would at least consider feedback/amendments from you in the first instance. At worst they can reject the proposed amendments and then you will have to decide to accept or look elsewhere, but at least you would have given it a go.
Contracts obviously seek to impose clarity on a relationship and so I have some sympathy with a company attempting to create a completely black and white position (if it's not carved out, it's ours). If you are concerned about this approach and want more flexibility then you could revise so any work in your private time unrelated to anything work-related is yours. This comes with its own pitfalls in some ways - it's difficult to nail down with clarity where the dividing line is, which in part explains the company's desire for a black and white approach.
In terms of ownership of previous IP, it would depend on the wording of the agreement, they might just be looking for an assignment of future IP developed whilst working for them, or they might want an assignment of past IP not expressly referenced in the agreement. The latter would be rather draconian but that's not to say the company wouldn't request it!
I'd be glad to give a read of the contract on an informal basis, if you want.
These kind of details are exactly why you should hire a labour lawyer. Also, your 100% that if it is a share purchase (even if only a majority stake) that the old contract is simply still valid.
Acquisitions of any kind don't magically invalidate all the contracts the company has entered in over its lifespan.
That's the case in the EU with the Acquired Rights Directive (and national implementations) which covers transfers of businesses/undertakings. Employees will automatically transfer, provided that in certain cases employer can update terms and conditions/make redundancies where justified (so called ‘economic, technical or organisational reasons’).