Hi! Currently, our server runs numerous websites using commercial TLS certificates, which are going to expire within the next months and years. We would like to replace these certs with Let's Encrypt certificates while preserving the key pair in order not to break HPKP. What is the best way to switch to LE within these constraints? As a side note, all these websites were created before the upgrade to 3.1, so they're (presumably) still using the old vhost template. Thanks in advance!