Rename your old zone folder to zone_old . This way, if something breaks, you can revert.
If you already own the game and have missing files, use the Verify Integrity of Game Files feature in Steam to automatically redownload any corrupted or missing .ff (FastFile) zone data. cod mw3 all zone files download free
To download a zone file archive:
Navigate to the main menu and use the Manage Files menu to check if specific packs (Campaign, Multiplayer, Special Ops) are fully installed. Rename your old zone folder to zone_old