The event is finalized at the end of the month and exported with all other events for the month. When the next session of the event occurs, open the event and re-set finalization when asked. Proceed with the event as usual. Date the sessions correctly during the next month. It is also possible to change the event date to the start of the new month so that it will appear in the list of finalized events for that month (this is purely for your convenience and is not necessary).
At the end of the second month finalize the event as usual and export this event again. The sessions from the previous month will be ignored during the next update (as they have already been done last month).
Do not change the name of the event as it will be considered a new and different event.
Do not change the dates of sessions previously finalized as they will be considered new and different sessions in the next run.