Moving07, Im with you on this one. Funnily enough, I was in the same situation for a Taipei - Kuala Lumpur flight earlier this month, as I needed to change my flight to Malaysia to one week earlier. Matter of fact is, they actively ENCOURAGE online check ins as they make it clear that a fee will be charged if you check in at the airport. Moreover, there are options all over the browser window once you purchase your ticket online, and obviously you want to secure a good seat up the front (well I do anyway) by checking in earlier - this is all without sufficient warning that you are UNABLE TO CHANGE your flights once check in online.
I emailed my grievience via their website feedback as suggsted by the customer service rep over the phone, as I felt scammed like you. Luckiy enough, my online check in was cancelled one week later (took them awhile) and I was able to change my tickets for a fee.
My sister booked a flight with air asia a few days later and noted that there are now putting such warnings for people doing online check ins, which I am sure was NOT there before.
Anyway, I hope you get it sorted out.. good luck with that