The order stays with the merchant. The customer should be able to find it through a confirmation, their account or customer service. A new agent can get access if the merchant supports it and the customer grants permission. Checking a delivery status requires different access from changing an order or starting a return.
What does the switch look like?
A customer buys shoes with Agent A. A week later they use Agent B and want to send back one size. They find the order through the merchant link in the confirmation or sign in with the merchant. If the shop supports access for Agent B, the customer can first allow it to read the order. Starting the return is a separate step that can be approved or completed directly with the merchant.
What should merchants test?
Buy through one assistant and then try to find the order in the merchant’s account and through a different assistant. Check that the second assistant gets only the access the customer approved and that the customer can take over. UCP describes a merchant link for the order and distinguishes permission to read from permission to manage. The integrations involved determine which options are actually available.