You may see the error like below when trying to connect to old server.
no matching host key type found. Their offer: ssh-rsa,ssh-dss
sign_and_send_pubkey: no mutual signature supported
By adding the two options, you may be allowed to connect!
ssh -oHostKeyAlgorithms=+ssh-rsa -oPubkeyAcceptedAlgorithms=ssh-dss,ssh-rsa -i ~/.ssh/id_rsa user@an-old-host