I wouldn't buy the new edition if you have access to an older version. If possible, I would spend a couple of hours browsing through the new version and compare it with yours, especially if you know which discipline you've chosen for the exam. I would recommend looking at the appendices in the newer version to see if any new tables or charts have been added or modified. Erratas should be available for the version you have so that you can make any necessary corrections.
Like he said...the only thing that they probably change from edition to addition is the sample questions at the end of every section (just like they did in college so you can never sell your books back and always have to buy a new version).
The new editions are released usually because the reference material which the books are based on have changed or the exam material covered has changed. When I was studying I had a current edition of the CERM, but an older edition of the accompanying practice problems. Many of the practice problems were the same as the current edition CERM, but because the references on which they were based had been updated the solutions to those problems had changed.
At least for transportation it didnt. I had 3 editions while I was studying, and they were basically the same thing.
