you dont need to buy another license since you are just swapping your hard drive. (you can also transfer your license thru your windows online account as well. usually when changing motherboards.) best you create the USB installer from this site here
https://www.microsoft.com/en-in/software-download/windows10
remove all your SATA drives and leave your new SSD in.
boot. press F8 for the boot menu (or F2/DEL then go to boot menu). select the USB UEFI. proceed with the install. leave the keys blank. it will auto restart to your SSD to finish with account creation and login.
if you want to reuse your HDD as a secondary data drive. you can shutdown your pc. then attach your old hdd in. boot up. go to disk management. delete all the system/recovery partitions from your old OS. you can use a 3rd party software to merge the deleted partitions with the data one. (e.g. minitool/easeus partition master) go to windows explorer. right click on your HDD drive. go to permissions. have ONLY the following below listed and remove anything else thats not in the list. ok, apply to all subfolders.
SYSTEM
Authenticated Users
Administrators
Users
if you go to your settings. activation. you will see its already activated.
thats it. ez pz.