Personally I've experienced no conflicts in documents when moving from AOO to
LO 4.0/4.1 or back. But then I am comfortable moving between releases and
administrative installs for QA work.
Regards the design goals of a Sidebar, some good documentation worth a read.
Apache OpenOffice 4.0 release notes on Sidebar are here:
https://cwiki.apache.org/confluence/display/OOOUSERS/AOO+4.0+Release+Notes
an extract of the lead developers comments about how Sidebar was integrated
are here:
http://wiki.openoffice.org/wiki/Sidebar#Overview
And when it was picked up for inclusion in LibreOffice 4.1.0 as an
experimental feature, LO Devs have done some additional cleanup and rework
of the UI as described in the those release notes:
https://wiki.documentfoundation.org/ReleaseNotes/4.1#Other
Stuart