When changing certificates, you do not normally need to revoke the certificate. Normally you can just let it expire. Since Apache (and Nginx) let you declare which certificate file to use, you should be able to just change over to the new one and start using it.

That said, it is always a good practice to revoke a certificate that is no longer in use. I do know startSSL has a fee to do so, so you would want to contact them for that procedure. You might also want to contact startSSL just to make sure this complies with any rules or policies they might have though. We are not fully familiar with how they conduct they business, so it would best to confirm it with them.

Hope it helps,
Jason Colyer
DigitalOcean Platform Support Lead