I've got LAMP running on Ubuntu 14.04 and almost a year ago I installed SSL (TLS 1.2) certificate for my web server, so my site would only run under HTTPS protocol. On October 8 the certificate expires. And from now on I want to install and use Let's Encrypt. They have pretty good tutorials on how to use Let's Encrypt, so (hopefully) I won't have any issues with it. But I don't know what to do now. Must I wait till after my old cert expires? Or should I go ahead and proceed with Let's Encrypt installation ASAP? If the latter, then another question arises. How to get rid of the old cert? I mean, it's probably NOT a good idea to use two of them side by side. In short, too many things I don't understand.