Wirelessly posted (iPhone 4: Mozilla/5.0 (iPad; CPU OS 5_1_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9B206 Safari/7534.48.3)

No, there's no for you to do it unless you refund it and repurchase one. CX, like other Asian carrier, doesn't have such thing as low price guarantee.

Did you check the details of your ticket? Such change may be caused by Lower fare class as well which includes more restrictive ticket.