You can't change into an EC seat on the app. You can change from one EC seat to another EC seat, but the app doesn't have the capability to process the transaction needed to get you into an EC seat (whether or not you have to pay for it).

I have found more than once where TAs, res agents, and Sky Club agents see all EC seats as occupied even when there are seats open that will show on the app and dl.com and you can get them yourself on the web.