With the new pricing rule that doesn’t consider the date of the original booking in determining the price of the new stateroom, it is impossible to answer your question.
If the original booking was done early on Platinum opening day, it costs the lowest possible price for that stateroom. After that, the prices of the different categories go up as they are filled. If you then decide to switch, you are effectively booking a new room at the current price. The savings will be based both on timing of the change and the level of downgrade.