Well, Yoda;s line 'Forever will it dominate your destiny' does not necessarily mean youy can never change back, but you will always reap what you sew. Anakin paid, in the end, with his life. Falling is not something to be taken lightly; it's not a revolving door you ca keep going back and forth through.
But Luke did NOT fall, and that;s the point. A Jedi can lose his rag and not immedaitely fall- e.g. Obi-Wan in TPM. The step over the line for Luke would have been to kill Vader, but he pulled back from the brink JUST in time.