I think you are right, but I don't remember Dish doing this in the past... My memory may be from a long time ago; I don't remember exactly.
If Dish killed the ability to access your disk using that 612, then if there is any chance at all to get these recordings back, it will be by accessing the internal disk from your PC. Do you own this 612 or did Dish tell you to keep it once you unsubscribed? They claimed to be throwing away my 612's, but I had carefully copied everything I wanted off onto an EHD beforehand.
I may be able to help you read the programs off that internal disk, depending upon which channel they came from. Long ago, the last time I did this, there were no recordings on the internal disk that were encrypted. I believe subsequent to that time, Dish started encrypting programs from some channels.