The LNB was described by Uboatcmdr; but as far as the knobs that face the dish. You can have a dual LNB with one knob. If you have one knob and a dual lnb you will have to use a switch between the dish and the receiver so that you can get all channels.
As far as hacking, I don't know anything about it, and I don't know if they would take to kindly of its mentioning on this site. So that's all I can offer concerning that.
The reason I mention HD is because if you are going to upgrade to HD you will definately need the phase III dish, which your installer should know by that designation. Unfortunately, with the role out of Mpeg 4 decoding, you will need the newest dish on the market, which is the larger dish and is talked about extensively in threads about mpeg 4 rollout. It is being tested in Detroit right now and should be affecting us near the end of the year. You won't need it right away because there will probably not be any substantial gain for us New Yorkers. We already get our HD locals through the 3 lnb setup.
As far as not being able to change your subscription, this sounds strange. If you call up a rep at directv and talk to them, they can add it, and then you will have to pay for that subscription. Also, if you find the right rep you might be able to get some trial offer for a reduced rate. I know they were giving showtime for 6 months if you pay for HBO. So talk to a rep and ask them if there are any offers that they have that would allow you to get HBO at a reduced rate. Sometimes this takes calling more than once to find the right rep who knows what they are doing and the current offers.
This site is awesome as far as finding anything you need to know and several ways to do it. So, read up on some of the other posts and ask lots of questions. There are some really knowlegable folks here.
Peace