it’s just so easy to do a mass change there, you just go into your tags list and edit the tag itself, and it’s changed for all the relevant posts.
here on Multiply I started a tag sort-out ages ago, but it’s so tedious, so time consuming. yes, you can re-tag a whole bunch of posts at once in the media locker but you have to know which ones, and you have to tick the box for each one individually. the only way to get at all the posts that have a particular tag here is by clicking on that tag on your homepage, and then you get the posts in full, so it can take forever to go through them all.
have I mentioned that I really like blogging on WordPress? ;)