Moving from 3 HR companies to 1

We currently have 3 separate companies with 3 EINs. We are in the planning phase to move to one HR/PR company with three separate EINs (3 process levels). We have employees that move between companies and have issues with terminations in one company and hiring in the next company.

Does anyone have any experience in consolidating companies with separate EINs that they would be willing to share?

How do you record a transfer from one EIN to another and retain their history on HR11? We use absence management and use a benefit date to determine their accrual start date. So, I am trying to figure out what the dates should look like in the new company.

Any help/tips would be greatly appreciated.