It has nothing to do with an upgrade or the 2A forum being removed. Many of us posted very little in the 2A forum, if at all. I had 3,100 posts and have been reduced to less than 2,000. What sucks is that a TON of valuable information was lost with the click of a button, from thousands of contributing members across the board.
There's a "culling" function in the administration panel that allows for the removal of all posts up to a specified date. It could've been done on purpose or, it could've been done by accident by someone who didn't really know what they were doing. In any case, once the posts are "culled" (read deleted) the only way to get them back is a full database restoration and I can tell you for certain, that will never happen.
Some of this could've possibly been avoided had the move from phpbb to vBulletin occurred back in 2001.