When we reinstall a machine we used to enter via ssh, or change the ip of a hostname ssh doesn't allow us to log into the machine saying the key has change, then we need to edit the .ssh/known_hosts files by hand and remove the entry of this host. Ssh should warn the user that the host key has changed and give the option to allow the connection and automatically edit that file.
1) you can save and restore the keys when you reinstall (useful particularly if you have many clients). 2) See CheckHostIP in ssh_config(5) for the case where the address changes. 3) you can use "ssh-keygen -R hostname" to delete an entry from known_hosts rather than hand-editing.
This is quite deliberate, we want explicit user interaction to force a changed key. You can use "ssh-keygen -R [hostname]" to automate the actual removal, but we need users to *think about it*. This will not be changing.
Mass update RESOLVED->CLOSED after release of openssh-5.1