Is it possible to change the username for an ownCloud account? (Not the full name.)
no this is not possible. However you can create a new user with a new username and transfer its files via this occ command:
The title of that linked documentation says "Transferring Files to Another User", so most likely no.
Btw. a feature request to transfer calendars is available here:
a feature request for changing the username here:
If it is a single user, he/she can export the calendar/contacs of the old user-id and import it with the new one.