This is a common question, and my answer to a biochemistry student below applies to all my classes:
Many have asked if they have to buy the new edition of our textbook. They want to know if they can continue to use the old edition.
I am not officially supposed to answer "yes" to this question, but I am sympathetic. My experience is that USUALLY text book companies churn out "new" editions regularly to chisel money out of students, and this sort of injustice makes me angry, quite frankly. While there are a some pertinent new details, surely, I will try to mention them, and most of classic biochemistry (Kreb's cycle and electron transport chain and all that) will remain unchanged.
What is astonishing is how the previous edition always suddenly becomes worth nothing while the new edition, although it appears eerily similar to the old one, is worth hundreds of dollars.
If you wish to continue using the old edition, do so at your own risk--it will be up to you to figure out what pages I am referring to during class, because they won't be the same pages. So you have to routinely bug some patient person to look at their new edition to figure out where you are supposed to be in the old one. Also, be sure the homework problems I assign from the book are the same as those in the new edition.
I hope that helps,
Holly