Passphrases can be added to an existing key or changed without regenerating the key pair:
Note This will work if keys doesn't had a passphrase, otherwise you'll get this: Enter old passphrase: then Bad passphrase
$ ssh-keygen -p
Enter file in which the key is (/Users/tekkub/.ssh/id_rsa):
Key has comment '/Users/tekkub/.ssh/id_rsa'
Enter new passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved with the new passphrase.
If your key had passphrase then, There's no way to recover the passphrase for a pair of SSH keys. In that case you have to create a new pair of SSH keys.