Renewing SSL security certificates can be a little tricky on Mac OS X Tiger Server, but I've found what I think is the simplest way to do it, which is as follows:

1. On your server with the existing, functional certificate, open terminal and do 'sudo -s' (enter admin password) then do 'cd /etc/certificates,' then 'ls -la' - you should see 5 files for your secure site, such as:

www.servername.com.chcrt
www.servername.com.crt
www.servername.com.crtkey
www.servername.com.csr
www.servername.com.key

2. Make sure you've got the contents of this folder backed up! Then, in terminal, do 'less www.servername.com.csr' and copy and paste the signing request into a text file, then hit 'Q' to quit out of 'less'. Use this CSR to request your renewal certificate with your provider. Using your old CSR is almost always fine, and makes life a lot easier than generating a new one. When your order is finished, you should have yourself a shiny new certificate.

3. Shut down web service using server admin, and then jump back into terminal, and do 'mv www.servername.com.crt www.servername.com.crt-old' then do 'touch www.servername.com.crt' then 'pico www.servername.com.crt.' Paste your new certificate into the file, then hit 'Control + O' to save, then 'Control + X' to quit.

4. Start web service again with Server Admin, browse to your site using Firefox, control click the loaded page, and go 'View Page Info > Security > View' and you should see your new updated expiration date.

One thing I've noticed is that Server Admin stills sees the old expiration date on the certificate until the entire server is rebooted. But client browsers will see the new one, and that's pretty much all that matters.