My analysis is that a company should have an iterative process of for preparing the post merger integration.
1. when the letter of intention is signed, to start looking for detailed information about the target and start building a macro analysis
2. after the due diligence when more detailed information is gathered should be the second phasis to have a more precise view on the integration
3. at signing, to start preparing day 1 and the plan for 3 months
4. at closing, finalizing the integration plan with the 2 teams and communicate