Most M&A integration plans are meticulous about the first 100 days' communication calendar and nearly silent on the thing that actually determines whether the deal creates value: who has final say when the two organisations' former leaders disagree.
This gets deferred because it's uncomfortable to negotiate before the deal closes, and by the time it becomes unavoidable — usually within the first month — it gets resolved informally, through whoever pushes hardest or whoever has the CEO's ear that week. That informal resolution becomes the template for how the combined organisation actually makes decisions, regardless of what the integration plan says.
I've seen this go two ways. In deals where decision rights were named explicitly before day one — this category of decision belongs to this specific person, full stop — disagreements got resolved quickly and the organisation moved on. In deals where it wasn't named, the same categories of disagreement resurfaced repeatedly, each time consuming executive time and signalling to staff that the integration hadn't actually happened yet.
The uncomfortable part of this advice is that naming decision rights explicitly means someone loses authority they held before the deal. Deals that avoid this conversation to keep both sides happy in the short term tend to pay for it during the following two years, in the form of decisions that never quite get made.
If I could change one thing about how integration plans get built, it would be this: spend less time on the systems migration timeline in the first draft, and more time forcing the decision-rights conversation while the deal still has enough momentum to make it happen.