jquery script modification question

I'm working on a wordpress site that's running a plugin that uses a purpose built jquery script stored in the plugin folder.  This means that when the plugin is updated any edits I make to that script will be overriden.

Basically its a form field validation script and I want to add a few lines to it.

How can I override the plugin script with a new one that will survive plugin updates?..

I was thinking of creating a new script to include the old stuff and my changes and registering it & enqueueing properly from functions.php

before I do I thought I'd check with the experts to see if there is a well known solution that works better.

thanks