Calendars "Unmigrated" following update from Snow Leopard"

[Spectrum]

Hello there - I'm looking for some help. I have an Office Exchange account that previously was perfectly synchronising 3 calendars within the same account across 3 computers (all running Snow Leopard) and 2 iPhones (iOS9). These calendars were not iCloud calendars. iCloud was not being used. They were also not local calendars (they synced across all devices).

Following, or soon after, an update of all machines to El Capitan, I have observed that my sub-calendars are absent, and only my main calendar remains visible on the Calendar app, and on the iPhones.

On close inspection of my ~/Library/Calendars
In side there is a folder named: "Unmigrated"
A single folder inside has a cryptic 20+digit hex identifier with a ".exchange" extension.
Inside contains a number of similar folders with hex identifiers with ".calendar" extensions.
Within each of these there are info.plist files, in which I can find the string name of each missing calendar.
Moreover, the Events folder inside each .calendar folder contains hundreds of ".ics" files. Using QuickLook on these files reveals them to be individual entries to my calendars that are now missing.

Question: How would I go about reinstating these files into a working calendar?!!? The data appear to be all there, but for some reason were not migrated during the upgrade(s).

Also, rather perplexingly, I don't understand why copies of these calendars weren't also held on the server. It seems like the migration process caused them to be deleted off the Exchange server...hence not on my iPhones either!

 

[Spectrum]

OK - I solved this. Inside ~Library/Calendar was a folder named "Unmigrated Calendars".
Inside there was an exchange folder with a list of all the missing events.

I selected one and double-clicked - and was able to select a calendar to import it into.
I then selected all the remaining events (500+!), and double-clicked. I then had a popup appear for each event, and just held down "Return" until they were all imported.

Finally, despite being imported into an Exchange Calendar, they were still not synchronising. To force this, I had to go month by month and copy each month of events from the imported Calendar into yet another new Calendar. Now they synchronise across all devices. Possibly this could have been achieved by merging the imported event calendar with another calendar, I'm not sure. (It would be a safer/quicker solution, but I didn't know about it at the time).

I post this info here in case someone else every has this problem.