In order not to lose the remaining time for a Certificate that is close to the expiration date, you need to make sure to purchase a renewal certificate and generate a new CSR.
We strongly recommend not to use the original CSR code used during initial certificate activation.
Please make sure to purchase and activate a renewal certificate and not a new one – this will also help you deal with the validation process in case you have an OV/EV certificate.
NOTE: renewal does not happen automatically, you need to generate a “renewal CSR” code plus an RSA Key, follow up the process similar to activation and then reinstall the Certificate on the server for the renewal to take effect.