Changing Calendar Events deletes

[Rabina]

Can anyone tell me how to avoid the forum set up from deleting people who have signed up for a calendar event when I change the event's time? I had to amend one of our running group appointments and when I edited the times it simply deleted all of the attendees and I had to go to all the hassle of re-informing those who had signed up previously (whom I could remember) to sign on again and register their attendance.

Thanks for your tips.