After having lost a few times the long work devoted to modify a style, I found that a very safe way is to start making a copy of the style I'm starting from.
Then, I work on one variation at a time, making modification of one variation, exit from Style Edit, play to check if the modification is ok, goes back for further tweaking (if it is the case), play again. And when the variation looks fine, save the COPIED style.
Then do the same on all variations, intro, ending, fill in, break, saving each part on the copied style.
This way the risk of loosing the work done is practically null.
Ciao
Angelo