DKIM selectors are designed to let multiple keys coexist.
Before cutover
Publish the new provider’s DKIM record or CNAME and verify it resolves correctly.
During overlap
Allow the old provider’s selector to remain published while old messages can still be in queues and while traffic transitions.
Verify delivered messages from the new provider show a valid signature with the expected d= domain and new selector.
Retire
After the old sender is fully decommissioned and its signed messages have had time to clear normal delivery paths, remove the obsolete key according to your operational policy.