A transfer is a mutual confirmation: the old account requests, the new one accepts. The request needs the transaction ID of both accounts — the order numbers from the developer registration fees.
The transaction ID was rejected
Official guidance said to strip the prefix from the order number. In practice it only passed with the prefix included.
Documentation and behaviour disagreed. When something is rejected, try both forms. A verification code from the account's inbox also works.
Back up before moving
After a transfer, financial reports are no longer reachable from the old account. They were downloaded first — they are needed later for tax filing and reconciliation.
A bonus: the commission halved
While reorganizing, the account was enrolled in the reduced service fee program: 15% instead of 30% under $1M annual revenue. Without enrolling, the full rate simply continues. Nobody prompts you to apply.