First of all, since Dish is letting you keep the 722, those recordings on its internal hard drive will still work just fine after it is deactivated. I have plenty of experience using my old deactivated 722k's for precisely this purpose.
You should be aware that the Hopper 3 would cost an extra $5 per month in fees ($15 Hopper 3 DVR fee, vs. only $10 per month for the Hopper Duo). Additionally, Dish would likely charge you a $100 upgrade fee up-front for the Hopper 3, instead of the low-cost (or no-cost) upgrade to the Hopper Duo.
I can tell you from my own personal experience that it is
very easy to do the upgrade from a 722 to a Hopper Duo yourself, especially if you are not adding a Joey. Just unhook the cable from the separator on the back of the 722, hook it up to the Hopper Duo, and run a Test Installation check. It is that simple.
Having said that, Dish would probably not allow you to do the upgrade yourself if you get the Hopper Duo from them. They seem to insist on sending an installer for any such upgrades. If you would be interested in purchasing a Hopper Duo so you can do the upgrade yourself, I have one that I am not using, that I can sell you
cheap. Send me a PM if you are interested. I also have a 1TB external hard drive that I know for sure will work with both a 722 and a Hopper Duo, if you would be interested in purchasing it also. I downsized my Dish system recently, so my extra equipment is just taking up space here, not being used. I would like to see it go to a good home.