I’m not talking about tumblr’s mass editor tool. It’s hardly useful if your blog has over, say, a thousand or so posts.
I’m talking about something to let you select all posts with a certain tag. For example, if I want to take all my posts tagged “.homestuck” and make them just “homestuck” because I’m starting to realize how bad of an idea the period was for fandom/shipping tags. I could just continue by using “homestuck” from here on out but that rubs me the wrong way for archiving reasons. Having half my Homestuck posts under one tag and half of them under another does not appeal to me in the slightest.