I'm exploring the ForgeRock Product Documentation and i'm not sure what's the difference between "Directory Services 6.5" and "OpenDJ 3". There was a re-brand and it's the OpenDJ was renamed "Directory Services", is that it?
ForgeRock Directory Services 6.5 is the continuation of OpenDJ by ForgeRock as a private branch (before that 99.95% of the contributions to OpenDJ were done by ForgeRock).
You can get a feel of the amount of changes between version 3 and version 6.5 by reading the release notes of all the ForgeRock releases. It's quite significant in 3 years.