If you push the days out, do you grandfather existing? That always comes up in these discussions.
I would like to see some type of annual award like airlines but still somehow keep your legacy level. Don’t change the tiers (except maybe add one more) but have something like Platinum+ or Diamond+ if you had 2 cruises in last year or 18 months. If you don’t have the 2 then you are just Platinum or Diamond without the plus. Then offer the pluses some additional perks - discounts on specality dining, WiFi, excursions or something.