Introducing Tag Replacer!
Tag Replacer allows you to mass-replace tags with ease. Install it, then go to Posts page on your dashboard of the blog you want, enter the tag you want replaced and the new tag, and Tag Replacer will do the rest.
Since editing posts put a lot of strains on servers, something I would like to avoid, operation of this extension is intentionally slowed down. So if you are going to replace tags of 4,000 posts, it’s better to just start it before going to bed.
Please note that it’s really experimental right now, so if you hit some bugs, please let me know by sending me an ask. Thank you!