Hmm, I can’t think of a good way of doing that. The compiler can do batch conversion from italics to underscore and vice versa, but that would only help you out with half of the job since it can’t convert underscored text to normal text. You can search by formatting in the Edit/Find/Find by Formatting... tool, but that’s a one-by-one step tool that would only save you a fractional amount of time.
Really this sounds like a job for OpenOffice, Word, or any other word processor that can do Search and replace for formatting via codes. Then you could do something similar to what you are doing by hand with a few successive searches.