I upgraded to a paid subscription but my calendars still show a warning.

When information about your payment arrives at WebCal.Guru, the new settings take effect immediately. The whole process rarely takes longer than a minute.

However, your calendar application does not know about your changes. And there is no way for WebCal.Guru tell your application.

To find out how you can force an update, please see How do I force my calendar application to refresh a calendar subscription? There's one notable exception - there is no practical way to make Google Calendar to refresh. The best approach is just to wait until the following day.