I phoned Dell to make sure this would all work. They checked with their techs, said yes, and because they sell hdd, I decided to get one like my current one (apparently a Seagate) for the same money as the WD (except Dell charges shipping). Figured any warranty issues would be easier thru them.
Then they switched me over to a tech supervisor, which seemed odd since I thought we were done, and he said that the only way to remove the OS on the old hdd was to reformat and lose all my data. I cant just remove the Windows folder, or uninstall W7Pro.
The only option is move my data to the new hdd (and if I've loaded W7 on it, I need to create a separate partition for this situation - my PC doesnt have RAID settings that allow two hdd with an OS on each to access each other). Then I could reformat old hdd to eliminate W7Pro...and move the data back if I wanted. But it defeats the purpose of my goal - downgrade W7 while leaving data untouched. If I wanted to backup a TB of data, I could buy an external, and I'm thinking maybe I should cancel the order and do that instead.
I guess someone would have suggested this if it were possible: make a partition on the old hdd for the data, then remove and/or reinstall the OS. Is the problem that the new partition must be initially empty memory (rather than contain the files you want to save), and that you cannot simply drag files from one partition to another?