Process
  - Know where your key is
  - Know how to generate a new CSR from that key

It's adviced to use a NEW private key, in case there was a private key compromise you didn't know about.

Also see https://www.ssllabs.com/downloads/SSL_TLS_Deployment_Best_Pr... (point 1.2).

BTW there is NO reason to regenerate the CSR if you reuse the private key.