So you can’t change code at random like this. If you need to change some behavior, then the plugin author needs to give you hooks – either filters or actions. If the plugin author didn’t provide you with the right hooks, then you can’t change the behavior, unfortunately.
You can see in the plugin code calls to do_action or apply_filters – those will tell you what kind of hooks the plugin developer put in place.
Your only “clean” options if you don’t have access to hooks, are to:
- ask the plugin author to allow some customization of the taxonomy used – she will decide how to best do this, through hooks or plugin settings, or…
- write or hire someone to write your own plugin
UPDATE
So since this is open source code, what you can do is fork the code, and then make your modification in your own copy. Make sure to change the plugin’s author and urls however, so you don’t mistakingly update the plugin down the road and erase your changes!
Hope this helps!