You see, mpdata is a local save file. Unlike modern games that store your rank on a cloud server (thanks, Blizzard/Steam), COD4 stored everything —your XP, your kill/death ratio, your weapon progress, your golden Dragunov—in a plain, unencrypted file on your hard drive.
C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\players\profiles\[YourProfileName] Backup your current file: Copy your existing to a safe folder on your desktop just in case. Swap the files: Move the downloaded Level 55 into your profile folder, replacing the old one. Fix the "Serial" Issue:
You unlock the final set of weapons, including the iconic G36C and the high-skill-cap M40A3 sniper rifle.
Users often download a pre-made mpdata file and replace their own. This often requires changing the CD Key in the Windows Registry to match the one associated with the downloaded file to prevent "corrupt data" errors.