Once you login to "My Account", you can select Equipment Information. It is essentially the same $199 offer that a new subscriber gets. If you have the HD Pack and Voom now with AEP, there is no programming price change in to go to HD Platinum. Existing subs qualify for the $10/month for 10 months as a 1st time subscriber to a DishHD package. They have a $100 offer if you turn in your 942 - I'd give you more if you aren't going to keep it.
The existing sub upgrade (DIU) comes with a $6/month lease fee on the 622. That applies even if it is the ONLY receiver on the account. If your 942 is currently your only receiver on the account, your monthly cost to keep both the 942 and 622 active would be the same as if you replace the 942 with the 622. If you have some 3rd receiver active, keeping the 942 would increase the monthly bill by its $5 Additional Receiver fee (I've assumed you OWN the 942).