I can't get the 'rename tag' function to work for me at all.

Is it possible that what you've been trying to do is merge tags, that is, change a bunch of the tags to another tag you've already used? I used to have an elijah tag and then when I started reading Lotrips fic that didn't have elijah in it I started using a lotrips tag. Then I tried to rename the elijah tag as lotrips and it wouldn't work. I think I had to go back, retag each individual lotrips entry as elijah, destroy the lotrips tag, and only then could I switch them all. (I'm sorry if that doesn't make sense!)