Employment records for successors, revised job descriptions, an organizational chart showing reporting lines before and after, handover notes, and correspondence in which others now make the operational calls. Client-facing records help too: who runs the account meetings and who signs off the work.
Show the handover happening, not just planned
A chart drawn for the file carries less weight than ordinary business records created while the transition occurred. Keep the emails delegating an account, the calendar showing supervision meetings, the approval threshold you set for a team lead, the contractor agreement. Assemble these as they arise rather than reconstructing them later.
Advisers can organize the collection and identify gaps; whether the record supports the classification is a legal judgment for counsel. One category sits outside the handover file and is easy to postpone: proof that the two businesses hold a qualifying relationship. Share registers, incorporation records, ownership charts and, where control is exercised otherwise than through shares, the instruments establishing it, all belong in the same collection.
Delegation evidence and corporate evidence fail independently, so a complete handover record cannot rescue an undocumented relationship. Hypothetical example: a pet-food manufacturer's founder assembles handover emails and approval thresholds within a fortnight, then finds the common ownership sits in a holding structure whose records are held by an accountant abroad and take three weeks to retrieve.