We have a Mac OS X server with an expiring certificate. We have renewed the certificate with the CA, generated a new CSR (using openssl on the command line) and have imported the new certificate successfully. The server admin certificates menu still displays the old certificates expiration date, and not the new one.
Assuming you're using Leopard Server (10.5), in the certificate section, select the expiring certificate and then click the "gear" button, and choose
Add Signed or Renewed Certificate from Certificate Authority... from the popup menu.
For more information, see Chapter 12 of the Leopard Server Security Configuration Guide.