No prompt to only send meeting updates to altered attendants

My client has a lot or reoccuring meetings, most of which created in the calendar of a shared mailbox. 

If she adds or removes attendants to these older meetings it does not prompt to only send the update to altered attendants, it will spam everyone attending the meeting.

I have created test meetings and it does prompt for these though. Our only option at this point is to delete the older meetings and restart them unless someone can suggest a fix?

Client is running up to date Outlook 365 on a fully patched Windows 10 machine

Thanks, 

Dave.