Force Cache Update

We're facing the following dilemma.

Our application requires a file to be included by our user's website like this:

<script src="http://ift.tt/2eI9G0H" async></script>

The problem: TTL for *.js files was set to be 1 year (.htaccess). This has already been changed (for example.js). However, all clients that loaded myfile.js before would not bust the file until one year from now.

Usually, you would simply add a GET param to force an update or change the filename. However, these solution won't work in our scenario, since we're not in control of the source code (and contacting our many users isn't a solution, either).

Unfortunately, our research on the web didn't reveal any solutions so far.

Can you think of ways to handle this issue? Or, do we simply need to accept the fact that setting TTL at 1 year was a big mistake?

Any hints much appreciated.