When Event is edited to Recurring, attendees are not updated
When already created Single Event is edited to Recurring, people who were invited to attend this Event and have accepted invitation are not updated about this change. The Event still appears in their Calendars as a Single Event, not Recurring. This is by design. If a single Event is created and attendees have accepted it, they agreed to attend this Single Event only.
After it's changed to Reccurring, it becomes different Event with different time consumption. So the only solution is to create new, Recurring Event and then let attendees to accept this new, Recurring Event again.