Going from a 622 to a 722 is a waste of time; the only difference is the hard drive. You're better off using EHD, which costs a $40 one-time-per-account activation fee, and you can get a terabyte external hard drive at Walmart for under $100 (more than quadruple your current capacity). This would help to indemnify yourself from Dish hardware failure, too. You can also use this to backup your recordings from the 622 before your next receiver upgrade.
Going to a 722K is worthwhile, though, because there are several key differences. Bigger disk (than the 622), newer hardware platform, learning remote, and two OTA tuners. Last time I called, it was $75 for the receiver, plus $40 for the OTA module, and it would extend my contract by two years with a minimum service level of Classic Bronze 100.
If you'd rather just buy it yourself, DishStore.net will sell you a 722k with the OTA module for $450. Once you have it in-hand, call Dish to activate it, and tell them to send you a box for your 622.