We in the digital age are very lucky that we have the ability to do this, I think - create word files side by side that are almost identical. Parallel drafts are a really good way to test out a major change before you make it. I ran into a situation recently where I was considering getting rid of a character (the brother) in my story. That kind of change has enough possible repercussions over the whole length of the draft that I wasn't just ready to "try it and see." By the time I'd tried it, undoing it would be very difficult, because finding all the instances of places where it had changed the text would be a nightmare.
So I created a parallel draft and added 'No Brother' to the title.
The disadvantage of parallel drafts is that you have to make sure you give yourself a way to keep them distinct. I suppose it would be comparable to losing yourself in a bunch of parallel worlds (world-lines!) and not remembering which one you really belonged in. This is the reason why I only very seldom work with parallel drafts, and when I do, I don't keep them distinct through numbering. For some reason, I can never quite remember which was the most recent number, and it doesn't orient me sufficiently well. I have to put big verbal cues in the file titles, and then once I've executed the change, make a quick decision about which draft is going to become my new "home base." That means not making any subsequent revisions until I've decided which draft is true. If I have to start inserting the same revisions in two different places it drives me nuts.
So if you happen to be in the position of considering a parallel draft strategy, here are my recommendations:
1. Don't do it a lot. Do it only for revisions with very broad consequences over the whole work.
2. Clearly label your parallel draft with a title that reminds you what you did with it.
3. Decide which draft is your home base before moving on to any other revisions.
After putting together my 'No Brother' draft I decided I liked it much better than the other, and the next re-titling that one received was 'Analog Draft.' So all in all, it was a successful endeavor.