I'm not sure Army is going to join the AAC. Why?
Army is currently playing about 2 P5 schools per year, 1 to 2 FCS schools, and the 2 other service academies Navy and Air Force. If they go to the AAC, they play 8 conference games, but what if the AAC moves to 9 conference games? Basically, Army's schedule would become 9 AAC games, Navy (out of conference), Air Force, and an FCS game. Do they want that schedule? Even at 8 conference games, the schedule becomes 8 AAC games, Navy, Air Force, 1 FCS, and 1 OOC game. Seems restrictive. Also, there is the cost of dropping future games and there could be some impact on the Army/Navy game.
I think Army would be better off with a 4 game scheduling alliance with the AAC. Thus, Army gets 4 guaranteed games per year, Navy and Air Force games, 1 to 2 FCS per year and scheduling flexibility on 4 to 5 games per year. Kind of like the Notre Dame deal with the ACC. In return, the AAC gets a guaranteed attractive opponent for 4 games per year which will help home attendance and TV rights value.