Here's how it goes. You used to only be able to do P1 and P2 in Abilene, then you would have to change campuses. Amarillo could be all 4 years, or just the first 2. Recently it changed so that Abilene is open for all 4 years.
In Amarillo, you pick where you do your last 2 years at your interview. In the fall of your P2 year, you have the option to change campuses, given that the campus you want has space. So if you were originally AMA/AMA, but you decide that you want Dallas for your last 2 years (AMA/DAL), someone else who chose Dallas would have to give up their spot. This does happen. I have 4 P2 friends who were AMA/DAL who switched to AMA/AMA. I also have a P1 friend who plans to switch to Amarillo instead of Dallas because she wants to stay with the friends she made here.
Now, normally when someone gives up their Dallas spot, another AMA person will take it. But with what appears to be an increasing number of people deciding to stay in Amarillo, I would assume that they would offer any excess Dallas spots to Abilene. So when you pick Abilene, you are choosing all 4 years there with a chance that you might be able to switch later. No guarantees.
Also, if you want to change your campus assignment, e-mail Mike Nall or Mike Schwettmann ASAP and let them know. The sooner you let them know, the more likely you are to get the change. There are a few people in my class that were supposed to be in Abilene and switched to Amarillo a few weeks before class started.