Putting this here as I always forget the commands when I need them! For more details see Gerts blog post linked below.
Commands
Create new ed25519 keys:
ssh-keygen -o -a 100 -t ed25519
Create secure rsa keys:
ssh-keygen -t rsa -b 4096 -o -a 100
Find all keys on your system:
for keyfile in ~/.ssh/*.pub; do ssh-keygen -l -f "${keyfile}"; done | uniq
Change or add passphrase to existing keys:
ssh-keygen -f ~/.ssh/id_rsa -p -o -a 100
Start ssh agent on headless server:
exec ssh-agent bash
https://blog.g3rt.nl/upgrade-your-ssh-keys.html
Upgrading SSH Keys