Actually I would be more tempted to think I could keep it if I did pay for it rather than if I received it for free because I have a compatible receiver.
Well, that gets back to dweber's question...
According to Scott the Hopper+ uses different software than the Hopper3. If that is the case I suspect that you would not be able to just unplug the Hopper+, switch the HDMI cable from the Hopper+ to the Hopper3 and continue watching Dish content. Perhaps Scott can comment more on this.
...Can the Hopper 3 go back to working as a non-Hopper+ receiver after a Hopper+ is installed?
I can think of a couple of reasons why someone would want to do that. First, suppose you have two Hopper 3's, and add a Hopper+ to each of them. Then, you decide that you want to sell a purchased Hopper 3, but keep its Hopper+ as a spare in case the Hopper+ for your first Hopper 3 dies. Would the new owner of your old Hopper 3 be unable to use it without also adding a Hopper+, just because you previously had a Hopper+ attached to it?
That brings me to the second (much more likely) scenario, what if your Hopper+ dies, and you don't have a spare? Hopefully, you could still use the Hopper 3 as a regular (non-Hopper+) receiver while you are waiting for your replacement Hopper+ to arrive. Otherwise, there are going to be a lot of pissed Dish customers if they lose all Hopper functionality just because an optional
accessory dies.
Getting back to my earlier point about having a pay-as-you-go account, it would be very inconvenient if I had to return the Hopper+ to Dish every time I deactivate service, only to have to wait for Dish to send me another Hopper+ every time I want to reactivate my Hopper 3. So, I sincerely hope that would not be the case.