I switched to Extreme HD also and still can't get FXHD. Get the message that I have to subscribe to the channel. When I received the Channel card in the mail, FXHD (553) in the DC Metro area is not even listed.
If you have HD Extreme package, and it is still showing that you are not subscribed, you will need to call up tech support and have a CSA check the codes on your STB's. Codes may need to be resent.