"Add correct host key in known_hosts" / multiple ssh host keys per hostname?

1. get the rsa key of your server, where server_ip is your server's IP address, such as 192.168.2.1:

   $ ssh-keyscan -t rsa server_ip
   

   Sample response:

   # server_ip SSH-2.0-OpenSSH_4.3
   server_ip ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAwH5EXZG...
   

2. and on the client, copy the entire response line server_ip ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAwH5EXZG..., and add this key to the bottom of your ~/.ssh/known_hosts file:

   server_ip ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAqx9m529...(the offending key, and/or the very bottom of the `known_hosts` file)
   server_ip ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAwH5EXZG... (line you're adding, copied and pasted from above)
   

Remove that the entry from known_hosts using:

ssh-keygen -R *ip_address_or_hostname*

This will remove the problematic IP or hostname from known_hosts file and try to connect again.

From the man pages:

-R hostname
Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts (see the -H option above).

A very simple way is:

cp ~/.ssh/known_hosts ~/.ssh/known_hosts.bak

Then edit known_hosts to clear the original key, then ssh to the host using:

ssh name@computer

It'll add the new key automatically; then compare the two files. A program such as meld is a nice way to compare the two files. Then merge the files to make known_hosts contain both keys

My 'reason' for keeping two keys is that the destination system is multiboot, even though I dare say there's a way of synchronizing the keys across the installations, it seems more straightforward to allow multiple keys.

EDIT 2015/06

I should add, revisiting it now, that I notice an even simpler way [as long as the entry is identifiable, normally from the hostname / IP address quite aside from the error message referencing its specific location];

1. Edit known_hosts to add # at the start of the 'old' entry in known_hosts temporarily
2. Connect [ssh to the host], agree to the prompt to add the new key 'automatically'
3. Then re-edit known_hosts to remove the #

There's even the option HostKeyAlias as in

ssh -o HostKeyAlias=mynewaliasforthemachine name@computer

then subsequently, after ssh client adds the new key under the alias, you may either edit known_hosts to substitute the 'real' hostname/IP address for the alias or connect to that incarnation of that host with the alias option evermore