Using WP_PLUGIN_URL will cause it to always serve files as http, no matter what the settings in WordPress are. Using plugins_url() ensures that a filter is applied when https is in use, and it will dynamically change the URLs accordingly.
Oops, I forgot about having to delete the existing definition. I was replying in between tasks at work and didn’t pay enough attention to your question sorry. Sounds like you figured it out thankfully 🙂