Do I need to reinstall Windows after upgrading my RAM?

No, Windows doesn’t care about RAM size changes—it’ll just see the new memory and start using it. Just check that your new sticks are running at the right speed and in the correct slots (usually slots 2 and 4 for dual channel) via BIOS or Task Manager. If speeds are mismatched, the system runs at the slower one, but still no reinstall needed.