I understand the 'normally' part as in "this is how it should be done." And yet in the roughly ten or so acquisitions I've been on the "inside" for (which is to say was at one of the two companies and was related either peripherally or intimately in the transaction from start to finish), every single one of them had some executive shuffling (aka separating) prior to the close.

Clearly, it is biased to Silicon Valley and my experience is probably different than someone who worked in non-tech companies, but if I put it into a rule of sorts I'd say; During a fund raising round executive positions are the most secure and individual contributors the least, during an acquisition these are reversed.