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.

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