As I recall, did you try this?
Renewal on a Site currently running an SSL certificate
Select the Internet Information Services Manager within the Administrative Tools menu.
Select the computer and web site (host) that you wish to secure.
Right mouse-click to select Properties.
Select the Directory Security tab.
Select Server Certificate under Secure Communications.
Click Next in the Welcome to the Web Server Certificate Wizard window.
Select Renew the current certificate, Click Next.
Select Prepare the request now, but send it later.
At the Name and Security Settings screen, fill in the [friendly] name field for the new certificate. Select bit length 1024-bit (Do you have 2048bit length?) length. Do NOT check Select cryptographic services provider (CSP)& Click Next.
Enter your Administrator contact information.
Enter a path and file name for the CSR.
Verify your request and then click Next.
At the Completing the Web Server screen, select Finish.
DO NOT REMOVE the pending request or the .crt file will not match and your certificate will not install.
Select Finish.