Can I Re-Use My Windows License or Product key again? and How
Can I Re-Use My Windows License or Product key again? and How
You're allowed to install it on one device legally, but they permit multiple installations during troubleshooting. In extreme cases, if both computers are active simultaneously and trigger a warning, the key may be invalidated.
When switching only the HDD to an SSD, think about cloning first rather than reinstalling. If that isn't possible or you prefer a clean setup, remove the old HDD (leave it as backup) and install W10 on the SSD. Leave the product key empty during installation unless you have a physical retail copy of Win10 Pro, then enter the key from inside the box. Once the installation is done, restart the system and manually run Windows Update. The software should activate automatically without needing any keys in Windows.
With an OEM license, you must not use that key on another device or different motherboard (refer to the guidelines below). With a non-OEM license, you may install it on up to five machines. This is the preferred method. Your Windows license "binds" to your motherboard. After reinstalling the OS and applying updates, the system should identify itself on the same machine and automatically perform a self-check via the activation service. You can enter the key manually since the automatic process may sometimes have issues.