At least initially some of the forum content was migrated. I don't know what the state of that is now.
AFAIK they migrated those posts where they could match an Oracle account with a Sun forum account. The rest were probably lost.
They might also have used a cut off date before which posts were archived somewhere.
One big "feature" of this new forum software is that it's rather hard to find things, especially historical things...
Nope, everything was moved. The posts that could not be matched with an OTN user account were merged into generic accounts. A few months ago many of the smaller Java forums were shut down and posts in them might have been removed. But the old sun.com links are definitely useless now.
Actually I am rather certain that "everything" wasn't moved.
There used to be a 'other' or 'social' forum (something like that) which wasn't moved or at least wasn't visible. Posting in there were along the lines of discussions about sporting events. There were a vast number of posts in there. I believe there were some also losses from older posts.