Hey there, @nadecker!
As long as you are using the same computer, the activation shouldn't be an issue. The operating system is tied to the motherboard, whereas the SSD/HDD are just a carrier for the system files. However, it's important to remember to unplug the HDD while clean installing Windows onto the SSD. Having more than one SATA drive connected could result in an OS confusion which might scatter the system files across all the storage plugged at that time.
Since you will be tampering with the storage configuration in your PC, it's highly recommended to backup all your important data from the system before you actually begin with the upgrade. Having a duplicate of your files from the HDD somewhere off-site will definitely keep your peace of mind and help you avoid any potential data loss.
Once Windows is up and running from the SSD, plug back the secondary HDD and re-format (after the backup) through
Disk management in Windows. Here's a tutorial you might find helpful regarding the
Windows Install & optimization for SSDs & HDDs.
Good luck!
SuperSoph_WD