So... There are two answers to this. The do as I say people, and the do as I do people..
The do as I say people say, The tank should be torn down and sterilized, then restarted with cycled media every time.
Purpose for this so far as I can tell is because of the risk of uranoma, as well as Brooklynella.
I dose formalin into the QT after copper has drawn down so I don't worry about Brook that much anymore. Uranoma has never really been a concern of mine.
The do is I do people, Will tell you to break the tank down between, but they just run one into the other...
I have had new bags floating, while changing over occupants.
How I consider it is when the quarantine is done, these fish are healthy. There is nothing transmissible any longer from the fish into the display, therefore, there should be nothing transmissible from the QT, to the new occupants.
This hasn't bitten me in the butt Yet.
after all, I have 7 empty tanks in my house atm... why not, why shouldn't I?
Should there be any issue running one qt into another? modest water change to clean up detritus? bit water change to cut Cu back down to half?