There is no general sequencing answer based solely on that departure. Have the actual proposed authority and supporting operation assessed before relying on a preferred date.
Keep the transition conditional where needed
Identify which duties would begin at each stage and which require the appropriate immigration step first. Report material changes before using the earlier analysis. A planned overlap period should be documented as a proposal, not described as work already authorized.
Work back from the date the transferee would actually hold the authority described, not from the date of the announcement. If the petition describes duties that only begin when the predecessor leaves, the file should say when that is and what the transferee does in the meantime. Where the overlap is long, consider whether it is better to file on the arrangement as it will exist at the start of employment and report the later change, or to wait until the structure is settled; that is a judgement about evidence and should be made deliberately rather than by default.
Hypothetical example: a lens-coating operations lead is due to move in the spring, but the approval limits transfer only after the autumn audit; filing in the spring means describing an interim role honestly rather than the eventual one. Keep the seven-year L-1A maximum in view when a company proposes to start the clock early for convenience.