In this case, because what you want to do is outside of what the TaskPaper engine looks for, you need to use some code independently of TaskPaper.
Fortunately, that is pretty simple to do. I will provide a link to some code in Ruby (already included in all Macs) that accomplishes what you want. Just read the whole discussion since they include some interesting points. And what you would be replace is the same text with the
** ** additions.
Search and replace using Ruby.
Ruby examples to replace text.
You could also use Atom and their REGEX search and replace the UPPER CASE WORDS to upper case words surrounded by ** ** . Just change ATOM from “case insensitive” to “case sensitive”. This has the advantage to let you know if your regex is okay or not. In your case, I don’t think the regex you provided will get what you want. Use regexr to test your regex.