Id immediately set the expectation that the process will be messy, take longer than expected, and require continued maintenance, iterations, and process improvements. Management usually tries to sell a transition as being great for everyone and will solve all problems. When it usually ends up being awful, painful, and take incredible effort. Disappointment is always better the sooner it is communicated. Align in principal for why an effort must happen and the realistic benefits to their daily life. Don't sell them a fairytale. I've found every transition is nost painful because expectations and communication is poorly managed.

I don't blame people. Usually the offenders are in a culture where telling the truth is unpopular. It just depends on if you want to have a successful transition, or make people feel good about a project that takes 6 years to not finish.