The current incarnation of AlternateHistory.com that replaced the technically much simpler Old Board began its life in 2004. Since then, it has existed and developed uninterrupted for over 14 years, under two different software platforms.
This was the vBulletin software incarnation of AH.com. It was referred to as the "new board" from 2004 to 2016, in order to differentiate it from its predecessor, the Old Board (active in 2000-2004, also known as the First Board). The Second Board officially opened on January 1st, 2004, and closed on May 12th, 2016. After the upgrade to XenForo software in 2016, the term "new" had become somewhat obsolete for this era of the board, as it was superceded by the newer software. Nevertheless, it's still part of the New Board era, even if not the current one.
The admin's intention to upgrade the discussion forum's software from vBulletin to XenForo was first announced in 2013. A test copy of AH.com running under XenForo was made available for viewing for several weeks, so board members could provide their user feedback. The plan was later temporarily shelved due to the admin's busyness, but it was clear that it would be eventually revisited. In early May 2016, Ian announced the move to the XenForo software and prepared several discussion threads to prepare board members for the transition, as well as for tech feedback after the move was completed. The transition was finished by mid May and the board is currently operating under this third software incarnation.
FYI: Shiny new AH discussion board at www.alternatehistory.net/discussion - Ian's (now-archived) thread announcing the launch of the New Board in early 2004, posted at the then still active soc.history.what-if online community.