OpenSSH since 5.7 prefers ECDSA over RSA and DSA keys, but you may get
errors such as:

    Error calculating host key fingerprint.
    ...
    The fingerprint for the ECDSA key sent by the remote host is
    ...
    Offending RSA key in /home/troll/.ssh/known_hosts:10
    ...

There seems to be no way to use the old RSA public key in you
`known_hosts` to automatically trust the new ECDSA key.  You'll have
to either copy `/etc/ssh/ssh_host_ecdsa_key.pub` into your
`known_hosts` file by hand, or do the usual
[[fingerprinting|fingerprints]].

See [Phil Pennock's (the Grumpy Troll) post][gt] for more details 

[gt]: http://bridge.grumpy-troll.org/2011/01/openssh.html