Moving Windows 8.1 to an SSD from an HDD

Hi, I bought a prebuilt PC and then added a new graphics card to it. I want to add an SSD, but the problem is that 8.1 is installed on my HDD. Is there any way of being able to reinstall my OS on the SSD without having to buy 8.1 again?

What do you mean it will reactivate fine?