Just from experience – in the past “change management” in projects (not explicitly merger, could also be e.g. SAP implementation) was somehow misused: it cost a lot of money, very often a lot of money and the benefits were not clearly measurable.
In the last projects I have done I did not use CM instead I worked with communication/ communication plans. With a clear, regular and interesting communication you can engage people, show benefits and results and support people to accept change. The ability to do change management (within in normal work, not at a separate task) should be a key skill of every project manager.