As soon as the transfer is contemplated, and preferably while the acquisition documents are still readily accessible. Reconstructing them later is the main source of delay.
Gather the transaction file while people still remember where it is
Three timelines interact. First, retrieving acquisition documents and predecessor payroll, which is fast in the year after a deal and slow once systems are decommissioned and the vendor's advisers have closed their files. Second, the qualifying year, which must be continuous, must be with a qualifying organisation and must fall within the three years immediately before filing; an acquisition mid-period narrows the room for error.
Third, USCIS processing, compressible by premium processing but not removable. If a client-site commissioning date is driving the plan, treat that date as provisional until the petition is filed, and make sure the contract governing supervision and control at the client's plant exists in advance. Finally, count prior United States time in L or H status against the five-year maximum before committing to a multi-year programme.