There are plenty of different threads about this, but the short story is this:
1. When you call D*, at the voice prompt, say "Cancel Service." This will take you to the retention dept.
2. Tell them what you want and that Cable/E*/FIOS/whoever will give you the same thing for free (if that's true), that you want to stay, but not for $300.
3. If it doesn't work with that rep, call back again a few days later. Eventually, you'll probably find a retention rep who is willing to give you what you want.
The dish itself will not be a problem. They'll put it in when they bring the new receivers - just make sure you tell them you need it.