Except in the UK you have Transfer of Undertakings and Protection of Employment (TUPE[1]) to protect you from being handed a 'new' contract on acquisition. You must not have worse terms than with your previous employer. You certainly cannot be forced to hand over IP which was not covered under the previous contract.
Employers bent on evil normally give you around 3 months then 'restructure' the company (i.e. give you a new job title) as a mechanism to say it is a new job and not covered. They still need to wait 90 days before they can enforce that and you have the option to reject it (but get your CV updated in the meantime because you're leaving).
Employment law varies enormously from state to state, country to country etc. People in the US used to At Will will have no concept of what it is like to be With Cause (and vice-versa).
However, even With Cause employers can get rid of anyone they want. In the UK it's classed as a 'right to manage your business' and the worst downside is a small compensation payment which rarely exceeds a few thousand pounds (occasionally there are large payouts for the most egregious things, especially racist or sexist bullying). In the US, payouts could be larger but for many businesses they just see that as part of the exit package and budget accordingly. Always remember, in a business, no-one cares about you and these people are not your friends.
Having been through it multiple times, my advice is always have your CV up to date and if you don't like the look or smell of the situation, it's time to leave. You will feel much better and less stressed if you feel in control of your destiny and not being played by some Evil Overlord who doesn't give a damn about you.
[1] https://en.wikipedia.org/wiki/TUPE