switch hdd of imacs

I do not believe that the hard drive on the iMacs are considered "user replaceable" but if you could, there is nothing special about the drives. You replace it, it will work just the same as the original.

 

If I understand correctly your intention is to take a harddrive with Tiger on it and place that into the aluminum iMac and the one with Leopard into the white iMac? Swapping the physical drive itself will not be a problem if you're comfortable fiddling with computer hardware. However neither computer system will probably work correctly because the drivers in the system folder that allow use of the hardware properly will not recognize the older/newer hardware correctly.