Book release day and then recheck on board, if prices went up 10% you win, if it's cheaper on the ship cancel and rebook there. You win/win and can't loose. If you wait till the ship only your Cat your Meal MDR choice may go and prices have risen fast on peak cruises like Alaska.
There is not a "regular fare" as Disney doesn't do the "sales" that some lines do (where basically rates are inflated first to make you think you are getting a deal). Opening Day is not a "sale" - those are generally the lowest rates will be (barring *GT, FLR, or military which *might* be offered post-PIF and may or may not be lower).
The OBB is 10% off whatever the prevailing rate is when you book. If the prevailing rate has increased more than 10% from Opening Day, then the OBB would not be cheaper
Example: Cabin type A is $2500 on opening day. You decide to wait to do an OBB, but by the time you sail, Cabin type A is going for $3500. Even with the discount for OBB it would be $3150, so still not cheaper.
Does that make sense? Basically it hurts nothing to book what you want on opening day if you know you want to cruise that itinerary and cabin. You can reshop it on board to see if it is lower. They will quote you the rate compared to what you have and only then do you say yes or no to the OBB.
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