The only options provide for the premium channels is to add if you don't have. If you have it there is a remove shown and if you click on it it says to call, there is no option to switch.
That's a shame. Like many good things, with "advances" in tech, they get lost.
Still a little confused, since I just finished a chat online with "James" [seems like this is a generic name they use for chat], who confirmed that this option is available!?
IIRC, that's 6-7 years ago, in your account you had listed the premiums subscribed to. And the remaining premiums. You simply checked a corresponding box, save and had access to that new premium channel. Then, reverse process, back to the original premium. Really nice!
Sure, if you have a cheaper than HBO premium, it wouldn't make sense [for DTV] to have this option enabled. But if you have HBO + 1 premium, would/should work.