There is always time when you have to decline meeting request.
When declining meeting request, the corresponding entry in your calendar is off course deleted.
The problem is when there is changes which make you want to change your answer but because the meeting has been deleted, it is no longer in your calendar and you will need to go to recover deleted items (dumpster) and search of it.
Well, good news a now you can decline a meeting request while keeping the entry in your calendar as free.
This functionality is currently being deployed, so it may not be fully available yet to your end-users (expected to be completed by end of November).
To enable this feature, connect to your Exchange Online mailbox using Outlook on the Web (aka Outlook Web Access) https://outlook.office365.com/ to access the Settings
Then access the Calendar\Events and invitations section to enable the Save declined events option
When declining request using Outlook Web Access, the meeting is saved in your calendar and shown as free. You can then later change your mind and accept it
Off course because you have declined the meeting, you wont get the notification before the meeting starts.
The option to keep declined events does not work with the classic Outlook client on Windows. OWA, new Outlook for Windows, Outlook for Mac and Outlook mobile support this functionality.
Exchange administrators have nothing to do to prepare but they can if needed disable this functionality using the calendar tenant settings using PowerShell (only once the deployment has been completed)
Set-CalendarSettings <tenant name> -EnablePreserveDeclinedMeetings $false