Complex sentence: Most of the changes were designed to phase in over a period of 4-10 years, and unless they are extended, will "sunset", or revert, at the end of 2010 to the previous laws.
Split into simpler sentences: Most of the changes were designed to phase in over a period of 4-10 years. Unless they are extended, it will "sunset," or revert, at the end of 2010 to the previous laws.