With this addon, you can filter threads by criteria then update tags for them.
Admin CP -> Applications -> Threads & Posts -> Batch Update Tags
View attachment 16222
View attachment 16223
HOW TO INSTALL
You can use the Add-On Installer of Chris D.
1) Download the zip file then Extract its contents.
2) Upload the contents of the upload folder to the root of your XenForo installation.
Login to Admin CP and using the provided XML file.