Trust relationships are not migrated after an upgrade of the CP server.
If you upgrade the CP clients after you upgrade the CP server, the installer recreates the trust relationships that were established by the client. You need not establish the trust relationships manually. However, the CP server and CP clients cannot communicate with each other till trust relationships are established.
If you do not upgrade the CP clients after you upgrade the CP server, you must recreate the trust relationships between the CP server and CP clients.