Renewals make sense in a per-certificate sales model where you receive a discount versus the cost of a "new" certificate. However, in our unlimited model the only real difference is that renewed certificates do not require generation of a new key or replacement on the server. It is a good practice to regenerate keys though, especially for multi-year certificates. If you have a situation where you really must renew a certificate, contact us.
NOTE: You can use the "renewal" feature in IIS as it actually generates a new CSR that you can submit as a standard request.