Hey McG,
You will NOT lose any functionality of the Events Calendar Pro plugin if you let the license expire.
You lose two main things:
• Updates for Events Calendar Pro, which means that your version of Events Calendar Pro falls out-of-date which can create problems if you update The Events Calendar and/or WordPress, which you should always do. Leaving WordPress out-of-date or using out-of-date plugins is one of the main ways your site becomes insecure and easier to hack.
• Customer support access here in the premium forums. You can read other replies and our knowledgebase articles, but would no longer be able to post for help about your own specific issues.
I hope this information is clear and helps! Let me know if you have any other questions I can try to help with.
Cheers,
George