Hi Sergey,
I agree that this is something to work towards. In my opinion (I'm not a developer, just a dabbler) it wouldn't be too hard to add the functionality, but it's the kind of operation that has quite a high chance of inadvertently breaking the wiki (by potentially changing lots of tiddlers and references all at once) unless it's done very carefully/cleverly - so then it probably becomes quite a challenging problem to think about (cue Jeremy).
Global search and replace in a normal editor will work, as long as you're sure that you're not going to change any of the code too (you can try your search term against a version of your wiki with no content in it, to make sure it doesn't find a hit)
Though I only think of it now, you should also be able to export all your tiddlers as a JSON file, do search and replace on that and then re-import it. That will probably work best.
Regards,
Richard