The HR20 requires the five-satellite dish (typically not referred to as an antenna.) The HR20 does receive MPEG4 channels, and that's the real motivation for getting an HR20. DirecTV will swap out the dish for you when an HR20 is installed. The HR10 will still work with the new dish (but won't see the new satellites.)
Yes, you'll have to change the dish, but as I said above, DirecTV will do that for you at no additional charge. Like the one you have now, the newer dish has four outputs. I'll be having this done on Sunday.