You can add or remove passphrase for your SSH private keys after creating your key pairs without creating a new key pair by using any of the following methods;

1. Interactive ssh-keygen

   $ ssh-keygen -p
   Enter file in which the key is (/home/user/.ssh/id_rsa):
   Enter new passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved with the new passphrase.

2. Inline ssh-keygen

   $ ssh-keygen -p -P old_passphrase -N new_passphrase -f /home/user/.ssh/id_rsa
   Your identification has been saved with the new passphrase.

3. openssl

   $ openssl rsa -in ~/.ssh/id_rsa -out ~/.ssh/id_rsa_new
   Enter pass phrase for /home/user/.ssh/id_rsa:
   writing RSA key