Although I am familiar with REBT, I do not know why the name was changed. However, I searched Google under the key words "'Rational Emotive Behavior Therapy' history" to get these possible sources: (Search within each one for "Rational Emotive Therapy".)
Possibly it is because he added D (Disputing irrational thoughts) and E (Evaluating the effect to develop a new way of thinking) to his A-B-C model.
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps. Thanks for asking.