Calendar events moved but the invites lost their attendees

Partly expected, and it is worth explaining why so you can set expectations with your users rather than chasing something that cannot be fixed.

A calendar event with attendees is not really one object — it is a set of linked objects, one in each attendee's calendar, coordinated by the organizing server. When you migrate the organizer's copy to a new platform, the new server has no relationship with the attendees' copies on the old system. The event data moves; the live coordination does not.

What reliably survives: the event itself, title, date, time, duration, location, description, and the recurrence pattern. What generally does not: RSVP status, and the organizer's ability to update the meeting for external attendees who were invited from the old system.

Practical guidance: for internal recurring meetings that matter, have organizers cancel and re-send the series after cutover. It takes minutes and produces a clean, fully functional meeting. For external meetings, the invite still exists on both sides — it just will not update if edited. Most organizations tell users to re-send anything recurring and important, and to leave one-off past events alone entirely. Trying to perfectly reconstruct RSVP state across platforms is not achievable and not worth the effort.