It’s disconnected at once. Revoke certificate asks for a reason (Key compromise, Superseded, Device retired or Unspecified) and for the device serial as confirmation. The fingerprint goes into the revocation set of every gateway node, the live connection is dropped, and the next login attempt gets CONNACK 5. It’s also listed in the next CRL; each issuing CA’s list is valid for 7 days.
It can’t be undone. The device needs a new certificate before it connects again.