I do believe that integration planning should begin in the early stages, perhaps at a high level during due diligence, and then evolve as the acquirer gains a deeper understanding of the target. Some elements can even be co-created to build trust between teams. However, real execution can only begin once the deal has closed. Starting too early can lead to misalignment or decisions based on incomplete information, while starting too late can create confusion, slow momentum and make relationship building more difficult.
The best approach is to outline the plan early on, refining it as the deal progresses, while also beginning to foster connections between teams, so that integration feels natural by the time it is needed.