To read quickly through the updates in a new edition of an old book try this method.
Read the preface and foreword to the new edition. The good authors make it a point to tell us where they have added, where they have changed, what they have deleted, what they have modified etc.
All important changes in the new edition is known by this method. Changes not listed or mentioned will only be simple editorial changes like sentence formation changes or change in words.
Refer the chapters where changes have been specified and read it preferably by borrowing the new book from regional medical library or college library. Make notes of the changes and insert it in your old book. So you can prevent investing 1000s of rupees on newer editions by using this common sense technique.
Another thing to be stressed is that do not let the new editions scare you. With this method u can revise the new book even if it is available only 2 weeks prior to the exam date.
Buying new book may not be possible always (economic or otherwise) and the new book may not be available in the library we have access to. In that scenario aleast visit the local book shop and read the preface and foreword and see the changes specified and try to locate them and note them in your memory especially if they r in some chapter u know is important for PG entrance. Better still you can photocopy the preface of new book (photocopying the preface is not a violation of any copyrights act).
Never go to an exam without even seeing the new edition book, some times certain small changes becomes detrimental when questions are asked on it e.g IBH
So watch out for new editions and do see them before the exams. All the best for your success.
Note: As the new editions of the major subject books are due this year, this useful piece of advice might allay some of the anxieties of the Indian PG aspirants who are perennially short of time.