That said, I once had exactly the described problem: post-acquisition, new company wants to adjust a lot of contractual wording on things like IP heavily in their favour, at a software business where many of the staff are also creative outside work in one way or another. Most of my colleagues didn't realise the implications of the proposed IP clauses and in particular the potential impact on their time outside office hours until these dangers were pointed out, but many strongly disliked the new terms once awareness was raised.
Without getting into details I possibly shouldn't, let's just say that what the acquiring company's lawyers or HR people would like to happen will probably be outweighed by a significant proportion of staff from the acquired company refusing to sign the oppressive deal and threatening to walk. If you can reach critical mass, management is likely to step in and do what they have to so they can protect the new investment and CTA. In the end, the wording of the relevant sections in our new contracts was identical to the corresponding sections in our old contracts.
Incidentally, probably one of the biggest mistakes of my professional career was sticking around for too long after I already knew what kind of business the new employer was from their initial behaviour. With hindsight, I should have given them a fair chance once they'd backed down -- a few months, perhaps -- but then having confirmed that the new corporate culture was similarly unwelcome in many other respects I should have started looking long before I actually did. YMMV.