I think the point, which I agree with, is that the original Princess Plus Beverage package included the Premier Beverage package - at that time there was ONLY the Premier Beverage package.
However now that there are two packages (Plus and Premier) anyone who had booked a cruise with the Plus Package (with Premier Beverage Package) has still the Plus Package but the drink aspect has been changed/downgraded to the Plus Beverage plan from the Premier Beverage plan.
Hopefully any historical bookings (i.e., made before the change) will still be honored with the Premier Drink plan