Do I need a new ISBN for every version of my e-book?

No. E-books don’t need ISBN’s at all. Remember that ISBN’s are essentially universal (at least in the bookseller world) stock numbers. E-books won’t ever be inventoried or accounted by the copy, so no ISBN number is needed. Whoever you publish with, such as Kindle, will have their own internal project numbering system or other method of controlling their internal database. Your book will get assigned some sort of tracking number. But an ISBN? No.