I have approx. 20 Dell OptiPlex of the same spec and pre-installed with Window 7. I have configured and installed a basic working environment on one PC and plan to deploy the image to all remaining 19 PC. Since the PC do not have the conventional product key sticker anymore. How do I extract the Product key from other PC to update the Window license or will that work without any update since the Dell OS is already pre-licensed?
Because you "technically" cannot clone/image OEM installs, you have to manually activate each one (cloning/imaging is usually reserved for VL/enterprise installs, which uses a single key).
However (1), Windows 8+ will usually check the BIOS for the key and activate (or at least pull the key and require you only to click on the Activate button).
However (2), since you are downgrading to Windows 7, you could use whatever key you are using to activate Windows 7 for all of them.
However (3), if you are cloning a Dell factory install, it will likely include a key that cannot be used for manual activation and "may" not be clone-able, triggering Windows activation after booting to Windows the first time.