AMC may have looked at ratings trends in general and decided that regardless of what they try, they're probably not going to get back to beyond a 2.6-2.8, hence they get rid of the older actors who are costing them the most money, such as Brooke, Vincent and etc. Eden they probably (and wrongfully IMO) wil help with 18-24 demographics and the vocal (in the past, yes) lesbians.
Its one thing to pay alot of money per ep for a show thats only getting 2.3 in the ratings, but its bearable if you're paying bargain basement rates. The question is, how long are the affiliates going to put up with it before they decide they can get more bang for the buck somewhere else.