SSH Connection Failed: AWS Cloud Access failed after switch IP

My Case:

Server OS: FreeBSD 8.x
After switching IPs between 2 application, the new server can be ssh, but the previous one failed.
New : x.x.x.143
Old : x.x.x.250

SSH Access command to new server ok, but the old one failed.
I ask my bos, and luckily it not because of wrong configured, but the access cache that stored in host..

1. Run this command:
   % ee .ssh/known_hosts
   
   This file keep cache of previous access ssh command
2. Delete all the content
3. Save
4. Try run the ssh command again:
   %ssh -i cloudcert.pem os@x.x.x.250
5. Connection success!.