I got $10 off per month for 6 months and free Showtime for 3 months. This was without a contract extension. They offered MRV installed for free (a $200 something value) if I entered into another 2 year contract. I declined for now.
The only way I know to get a HR24 on an existing account is to purchase it at a store at around $200. Get all your deals that a 2 year contract extension can get you, because activating a new box will automatically give you a 2 year commitment. The HR24 sounds cool and all. I have a HR22 that can surely slows down at times. However, there was a thread around here talking about lag on the HR24 as well.