Almost three years ago, my employer was acquired by its major competitor. Having been through a very similar situation at a previous employer that ended in a fairly nasty fashion, this initially had me rather worried. Now, having had time to watch the intricate corporate dances play out, I can see that the two situations are chalk and cheese, and I'm rather happy about that.
Along the way, I learned a few lessons. I learned them from talking to other people involved, on both sides of the corporate fence and from my own experiences.
Most important, I think, is to embrace change. You're going to get it, whether you like it or not. It can be very hard work to remain positive in times of corporate upheaval, especially if some of your colleagues are incurable cynics and doomsayers, but in the long run it is worth it.
The next most important thing, in my opinion, is to engage with your new peers. Don't just listen passively in interminable phone conferences if you can avoid it. Get over there in person and talk to them! Stop thinking of them as competition. They're people, just like you, and it is extremely likely that each side can learn something from the other.
One of the first serious engagements I had with my new peers was a meeting to talk about the deployment of the acquiring company's standard operating environment for corporate desktop PCs.
I vividly recall attending a phone conference with my manager and a group of tech and managerial staff from the other side. My manager and I were on the receiving end of what seemed like an unreasonably hard sell, and given our relative inexperience with large-scale desktop management, we were actually extremely eager to adopt the desktop standard that was being pushed at us.
At one point I politely interrupted the meeting and made it crystal clear that we were just as interested in it as them, since desktop management was something we'd never really succeeded at to any degree. We wanted them to do it so that we could get on with our real work! The SOE proponents seemed thoroughly shocked, but with that part out of the way, we got down to details that actually mattered. The sailing was much smoother after that, and the end result thoroughly positive.
A nice side-effect of embracing change, as we did with desktops, is that it helps your new peers to see you in a positive light right from the start. You sure don't want to be thought of as that toxic old guy in the corner who hates anything new or different. First impressions are lasting impressions.
That pattern has repeated several times over the last three years, and the learning has continued on both sides. There have been exceptions, of course. There were people (from both sides of the acquisition, it must be said) that appeared unable to fully accept the reality of the acquisition. That's fine --- it was their choice. Just don't let their choices ruin it for you.