The details are lengthy, so I'll provide a link
to the main page. There are further links to FAQ and other sections from this page.
In a nutshell, HAL is replacing a program that had few benefits for repeaters with one that has more amenities--and revamping member levels in the process. Six levels (25, 50, 100, 300, 500, and 700 nights) are now replaced with four (1+ cruise completed, and 30, 75, and 200 "Cruise Days.") Cruise Days are based on the length of the cruise, onboard spending amounts, and a few other factors.
A few of things I noticed right off the bat. Medallions for 100+ days are no longer awarded. There is no special recognition for those with 300+ nights (and there are many of them). The onboard spending that gives extra Cruise Day credits will mean that pax can get to upper levels quickly--I don't know if that's a good thing. Also, it used to be that hitting a new level during your current cruise would take effect that cruise. No longer.
I do like the amenities, though. Not that I need them to sail HAL.