As part of the migration from classic Hangouts to Hangouts Meet that Google announced last year, they will begin migrating video conferencing for recurring Calendar events from classic Hangouts to Meet starting on March 25, 2019.
If you have a recurring meeting that is impacted by this change, you will see a migration banner notifying you that future occurrences will be on Meet. After the end of the call, all participants will get an updated event information with the new meeting details. Single instance (non-recurring) Calendar events will remain unchanged.
However, if you have a complex Calendar event setup, your recurring meetings may not automatically update to Meet. In this case, you'll see a banner in the video call notifying you to update the Calendar event to Meet
For additional support, visit Migration FAQs.
Source: G Suite Updates