How to change SSH private key passphrase

From time to time I have to update passwords used to secure private keys to keep myself a bit more sane.
I will demonstrate simple and straightforward way to accomplish this task.

Set new passphrase interactively.

$ ssh-keygen -p

Enter file in which the key is (/home/milosz/.ssh/id_rsa): /home/milosz/.ssh/knowledge_base_key
Enter old passphrase:                           **********
Enter new passphrase (empty for no passphrase): **********
Enter same passphrase again:                    **********
Your identification has been saved with the new passphrase.

Set new passphrase interactively for specified key.

$ ssh-keygen -p -f ~/.ssh/knowledge_base_key

Enter old passphrase: **********
Enter new passphrase (empty for no passphrase): **********
Enter same passphrase again:                    **********
Your identification has been saved with the new passphrase.

Set new passphrase interactively for a specified key with empty passphrase.

$ ssh-keygen -p -f ~/.ssh/knowledge_base_key -P ""

Enter new passphrase (empty for no passphrase): **********
Enter same passphrase again:                    **********
Your identification has been saved with the new passphrase.

Set new passphrase non-interactively.

$ ssh-keygen -p -f ~/.ssh/knowledge_base_key -P "oldpass" -N "newpass"

Your identification has been saved with the new passphrase.