I've been part of 6 separate buy-out/merger/acquisitions (from both angles). In year 3 of current one.
The event, whether you are acquired or acquiree, presents significant and unique opportunities to move vertically and/or horizontally to improve your situation. If you're sitting there wondering about job security then you (a) didn't have job security in the first place; (b) are wasting everyone's time, including yours.