Modifying text in Subscriber Cancellation

When a user who has subscribed to a recurring plan using PayPal cancels, there is some text that appears instructing them to also visit Paypal and cancel their recurring profile there (“Please log in at PayPal to cancel your subscription.”)

I’d like to augment that message with some additional instructions. Is there a way to edit this text? I see it in a few files:
includesgatewayspaypalpaypal-responses.inc.php
includesgatewayspaypalpaypal-cancellation-in.inc.php
includesgatewayspaypalpaypal-cancellation-pf-in.inc.php

But it looks like these files could be overwritten by plugin updates. Is there a safe place to modify this message?

Thanks,
David