The main changes are in appearance (ie, there's new icons to show which forum has new posts since you last visited and which doesn't, but obviously if it's been 6 months since you logged ever single forum is likely to have new posts).

There are always a few forums that get reorganized due to mergers or due to someone coming up with a better idea. For example, the LAN forum has been renamed LATAM because the airline renamed itself (after it merged with TAM). Depending on when exactly you last visited, if US Airways was still an active brand then, the American Airlines forum may have looked a bit different since US Airway was still semi-separated but is now completely merged in. The Rental Car forums got "simplified" with only a few rental companies having their own separate forums and then a main "rental car" forum was created both for catch-all rental car questions and for the remaining rental companies. Etc.