Judith did re-work Whitney My Love and re-release it in hardcover. Most of the book is the same. The parts that were updated were scenes that, in modern times, had become "controversial". There were also a few other tweaks made that were less obvious unless you've read the book dozens of times (#guilty). To be honest, I still love the old version best. Even though it might no longer be politically correct and may receive criticism, I find that Clayton's actions in the first book were part of what made it so emotional and soul-rending to me.
However, the new version DOES have a very extended epilogue that provides some backstory on Stephen and, thus, paves the way for Until You. So for that reason, you MUST read the updated version!! I usually read the original WML and then switch over and read the epilogue before moving on to a reread of Until You.
Hope this helps! <3
-- Jessica