TiddlerTool is my newest attempt for a general purpose tool that lets you search and replace anything. So far it lets you add tags to search using tiddler titles, field names and tags then operate one the results or select a subset of the results to act on.
It can add, remove or replace/rename tags and fields. If you click on a tiddler title, field name or the contents of a field in the search interface you can edit it. You can also add tags to tiddlers individually using the same interface as when you edit a tiddler normally.
Renaming fields has a few problems that I haven't tracked down yet. The editing interface for an empty field opens for each tiddler where you rename a field. I don't know why. Also if you change the names of fields for all tiddlers returned by your search the field contents is lost.
For some reason if you rename a tiddler using this the renamed tiddler won't always show up in the list until you reload the wiki. I have no idea why. The table can also become wider than the tiddler.
It is packaged as a plugin on the demo site
here.
There are other things, but the options and everything are hopefully easy enough to understand. Clicking on the question marks should open help pop-ups.