So you have an HD DVR you want to get rid of or wish to purchase one? OR both?
Anyway, keep the HD DVR in either case as it's the same price for an HDDVR as it is a regular HD receiver once you pay the DVR fee no matter how many DVRs you have.
ALso, call them up and tell them what you need and make a deal. You'd be surprised how far a little kindness goes when trying to deal with the CSRs.
If you're a longer term customer with a good payment history, you should be able to get a price break on what you need. You probably wont get it for free, but you most likely will not have to pay the full price.
Do expect that your contract gets reset to the full 24 months anytime you make a receiver purchase though.