Hello there!
I need your help to determine if the following is possible.

Background information: Our client was using several calenders in different mailboxes. For simplicity reasons we chose to combine all the information into a single calender, placed inside a shared mailbox. We created a new mailbox, added permissions for everyone needed and copied all the objects into the new calender. What we missed, is that some users were using personalized categories in their mailbox.
As a result, the colors in the calender went haywire. Now I know, that in order to use a shared calender, everyone who has access to it must use the same categories to prevent chaos with the colorization. So I changed and renamed every single category in the mailbox of the affected users. It seems to work fine for all the future objects in the calender.

Now one of the users came up to me and asked, if it is possible to restore all the categories/colors for the older objects, which were copied to the new calender. Problem is, we don't have any of the old data/mailboxes left. Is the necessary information somehow encoded in the objects itself? And if yes, is there a reasonable way to extract it?

Thanks for the support!