If you bought certificates prior to March 15 they are good for whatever OBC they had at the time you bought them. The changes are for Next Cruise certificates going forward.
They obviously want to kill the open bookings and focus on people making specific cruise bookings while aboard. Hence the changes to the OBC for those bookings, which reward cabin selection as well as cruise length. They are not worth the trouble for those booking an inside cabin for a 5-7 day cruise getting $25 OBC, but if you book a GS you get $200 for the same cruise. Also, note that you can't book anything under 5 days any longer.
You can see/download the pdf file of the new program here