New map, new options system, new scenarios, old scenarios were redesigned with diplomatic/military options, new cohesion system, new debarkment order, new music, better and faster IA etc..
Fact is though that I would have bought WIA at a reduced price but I'm not highly motivated for paying full price for playing a war I played a lot one year ago.
If I'm in the majority (and I have no statistical clue about that), AGEOD could financially benefit from a reduced price for BOA owners.