Except that with sat tv you aren't 'buying' anything, you are leasing/renting and never acquire ownership. In that scenario it is not unreasonable to expect that now and then you can rent new equipment that may better meet your needs or wants.
And as to what kind of deal can you reasonably expect? Well, if they want a new 2 year committment, then the deal you should be working from in negotiating is the same one a new subscriber gets. Note I said 'negotiating', you have to do that because their systems are set up to give as little as possible, and they have a slew of CSRs, Accountants and marketing types at their disposal to sway you to their position.
I always figure that at the end of the commitment period, if I'm happy with the service and the equipment, I'll stay on a month-to-month. But if there is a really upgraded piece of hardware that I think will fit me better, I'll go into haggle mode. Me offering 2-years of guaranteed income to them, they offering the equipment and some programming credits. Hopefully we'll come to some mutally agreeable position.
If not, then there is the I'm gonna cancel card. But always be prepared to do just that if you can't get a deal you think is good. It isn't unethical, immoral or illegal to do that, it IS good business on your part though. And remember, this sat tv service is just a business arrangement, nothing more, nothing less, no matter how many apologists or 'loyal' customers say the contrary.