You don't have to re-install your system from scratch when you move it to a new HDD or SSD, all you have to do is to use either SSD manufacturer provided migration tool ( as in the case of samsung SSDs: Samsung migration tool downloadable from their website) or use a generic yet powerful cloning software (under windows environment after you install it on your older drive) like Macrium reflect and you' ll be set in less than 20 minutes of time, your windows license will be valid as well as system HDD swapping is not considered "by Mr. windows" as a significant hardware change.
If not budget constrained go for a good quality SSD of 500 GB (Crucial MX500) or for samsung 860 EVO,
- if possible before you start this upgrade make a full (system and files) back up image of your system using built in windows back up tool or Macrium reflect (that you'll be using for cloning as well)
- install SSD in tower,
- boot your system, install macrium reflect home (free),
- clone your running system onto your new SSD (tutorials available online and on youtube) using Macrium reflect.
- After cloning process is finished, shutdown your system.
- remove older HDD from tower ( or just disconnect it's Sata power and data plugs
- boot your system: if successfull it'll be obviously running of your new SSD.
- When reconnecting your older drive make sure you know which is which before erasing it before booting your system as both drives being clones will be carrying same name (under windows), same system and both are bootable: better practice for the average user is to rename/erase older drive through usb/ after connecting it while system is already running to avoid altering the wrong drive, or best and more " classy" is to ensure in the Bios that the SSD is selected as the only boot device, then if in doubt double check device manager and in windows computer management> storage: where you'll be able to see each drive model, drive letter and name under windows (name that you could change [right click on drive under windows explorer and rename it] then proceed to erasing/editing your older drive to turn it into a storage). You could also -in case of an older small capacity HDD- keep it on the side as ready-to-use back up clone in case your system encures major issues in the future.
__How to clone your PC hard drive using Macrium Reflect >>
https://www.windowscentral.com/how-clone-your-pcs-hard-drive-macrium-reflect
__How to Clone Windows With Macrium Reflect Free>>
https://www.youtube.com/watch?v=U0PyZIqecII