I ask carlton for replacement my ab 900 a few weeks ago(i post it here too) and the result they directly sent me AB 2000 instead of AB 900 without my agreement.Now I sent back to them and ask another racket (AS-Ti), and my friend said they might be ask me to pay more about that .Do you guys have any idea about this situation?
Than you so much experts :)

Possibly. The AS-Ti is a more expensive racquet than the AB 900 or the AB 2000. However, the fact that they sent you an AB2000 as a replacement for your original racquet may mean that they will upgrade your racquet again. I got a 900 when I sent Carlton my defective AB600Ti, so they seem pretty willing to make the customer happy.