A while ago I wondered if it's possible to change the SSH passphrase as the phrase was rather hard to type and somewhat cumbersome. Till then I never thought about changing the passphrase and wasn't sure if it's possible, but it's actually pretty easy.
Run the following command to change the passphrase.
$ ssh-keygen -p -f path/to/your/keyfile
You will be asked to enter your old passphrase then you're free to define a new one. It's as simple as that.