You can have only 1 instance of a windows license on one computer at a time.
The license is tied to the motherboard, not the storage medium.
(Also it depends on OEM or Retail, retail has 3 shots).
Providing you kept the same board things should be as simple as they were when you first set things up.
You will need to format the other drive with Windows on it, so back up any data.
(You can do this during the install, it's one of the first steps).
It's recommended that you do a fresh install, but there are ways that you can clone your existing drive onto the SSD.
Don't forget to set the SSD to run in AHCI mode in the BIOS before installing to gain its maximum potential.