According to the tech chat it is like for like. In order to get the $200 rebate, you must have either a 921 or 942 owned or leased. You are eligible for the $49 211 upgrade with the 6000.
That's not what has been said at the Tech Chat and last month's Charlie Chat. The $200 rebate is for 921 or 942 users ONLY! Take the previous poster's advise and sell the 6000 on eBay. You will likely get more than the $200 rebate from the sale of the 6000 anyway.
You're better off selling the 6000 receiver on eBay as several other posters have stated. It makes no sense for E* to trade up a HD receiver for a HD DVR receiver for a $200 rebate.