call and ask is best thing to do. sounds like your eligible for an upgrade via a 2 year commitment. Through that the equipment would be very cheap to lease if anything, and if you only have 1 recvr on the acct there is no receiver fee to worry about. HD programming options are 10$/month pay as you go, 99$ one time fee or autopay and paperless for HD free for life with a different 2 yr commitment. You also have the option to just have the HD equip with no HD for no charge obviously. I wouldn't upgrade unless you have the HDTV ready, HD recvr + SD TV = worse quality SD. They will replace your 500 twin with a 1000.4 more than likely too. HD requires signal from 3 satellites. 500 only capable of 2.
edit: and you would definately want the new sat. if you did something like deny a new dish, they would install the recvr and when you wanted to activate HD you would miss your channels till you got the new dish.