I've been part of two large acquisitions with major culture changes. In the end, as long as the acquirer has a strong culture, and the management sets clear expectations, it will be just fine. I find field staff figure it out faster than those soft people in the office, but a certain amount of attrition is a natural part of the process, and honestly not a problem. That's the best way of gaining "efficiencies", through headcount reductions.
When TAQA bought Northrock, Primewest, Pioneer Canada and Shiningbank all within "effectively one year" it was a total disaster, but then again, TAQA had zero local employees before that. A case study that should be taught in MBA school.