Next time, I'll turn my phone off before deleting the account in the hope that it will not turn my calendar events into "default".
As a workaround, I changed my desktop manager to sync one way, from Outlook to the BB, and synced only future events back to my BB. I ended up having to delete the duplicate "default" events, but it didn't take that long. Then I changed the desktop sync settings to my regular ones and all is well.
Now my Today theme shows the calendar events again.