Having been in Chris' shoes (being acquired by IBM) I found the reactions of different people in the company most interesting. Several people had the same sort of "Everything is different now that IBM owns us." reaction, but if you unpack that, the reality is a bit different.
For a company like Redhat I can't imagine it will be largely changed in the first couple of years at least, so for that period of time you will be working with the same co-workers, and if you're not in the Redhat C-suite probably for the same boss in the same facilities on the same stuff. So in that regard nothing has changed.
What will be different will be access to a company that has been running for over 100 years and evolved over that time, at all levels. Facilities all around the world. Access to some of the scientists that you may have only read their papers but now can chat with via slack.
If you're currently an executive staff level type (VP, Exec VP, etc) and you're part of the acquisition, you're going to be landing in a whole different world of agendas and counter agendas, that can be tough to navigate. You will want to establish relationships quickly, they will be both rewarding and help you see things that might be coming your way that you would miss otherwise.
At its heart though, IBM is just another company trying to do things to make the world a better place and to profit off helping the world get there. You will find the Finance guys are ones who have the ability to make things happen or not, the marketing folks may often be distracted by shiny objects that don't seem to contribute to the vision, and some of the fading parts of the business are not going into the long goodnight easily.
And if you're patient and look clearly, you will see that fundamentally the company bought you so that you're technology could both improve the company and continue to grow in its own right, and so everyone is "on your side" to a large extent. The biggest danger to your dreams will be your own executives, not IBM. They are the ones who have gone from being masters of their own universe to a star in somebody else's universe, and sometimes that transition affects them in ways that isn't immediately apparent.