Hi Bob,
Thanks for reaching out to us!
That’s a great question there.
You have the choice not to update your WordPress installation to keep compatibility, but in the long run that is not recommended.
Otherwise, if you would like to update the calendar plugin, then yes, you will need a valid subscription / license. This will grant you updates and premium support until your subscription is valid.
Hope this clarifies. Let me know if you have any further questions.
Cheers,
Andras