It needs to be graphical. No sed, awk, grep, perl, whatever. I know how to use those and I do use them now, but I need to cherry-pick each replace in 300+ files.
I want
Lately Kate (if you use KDE) can do it, but in a very tricky way. Go to "Edit>Search in Files", and choose the folder within which your files exist.
The trick is that only after the search results appear, you will find a text box and a button called "Replace checked". This button will do what you want.