Finally got around to upgrading my OpenDS 2.2.0 to OpenDJ 2.4.3.
Clean upgrade - just followed the OpenDJ wiki.
Only change I had to make was from
./bin/rebuild-index -i dn2id -b "dc=example,dc=com"
./bin/rebuild-index --rebuildAll -b "dc=example,dc=com"
Rebuilding the dn2id index only left the sync-conflict index for my root backend in degraded mode.