Does anyone have any information as to the exact impact of this change?
Since it hasn't happened yet, there is no way to tell. RCI back-peddled from their initial plan in order to appease the large number of complaints from diamond members. For the smaller ships they'll also have a morning coffee and danish area set up, with a "concierge type" person on duty to help with tickets, tours, etc. The only thing really missing for diamond members on smaller vessels will be complimentary cocktails in the evening. They will be 25% off. Only wine (champagne too), and soft drinks will be complimentary.
For the ships with separate diamond lounges (Freedom, Liberty, Independence, and Oasis) it appears things will remain the same, with all drinks being complimentary, etc.
Why they decided to sell discounted drinks on the smaller ships, but continue with the freebies on the larger ships was not made clear. However I suspect profit motive played a role. It doesn't make sense to have this inconsistency, so look for it to become consistent down the road...as in 25% discount on the Freedom and Oasis class too.