Correct. After the ZCS 6 upgrade users preference was changed from 'reading pane off' to 'reading pane right' (three pane mode). I do not know what themes were in use as all users change their preference to suit
This has occurred on both a SLES10, RHEL and Ubuntu ZCS upgrade. And without the user setting the pane preference.
After the upgrade, 'reading pane right' (three pane mode) was set for all users. Consequently each user changed it back.