Price changes should take effect right away.
It's good that you remembered to change your Default Settings, but that only applies to new uploads.
You need to change your prices in Bulk Edit Prices as well.
Behind The Scenes > Bulk Edit Prices > Show All Images > Check the box next to the word "Images" ABOVE the actual images > Check the box next to "Select all ___ images on all __ pages" > Click "Edit Checked Images Simultaneously" > Click "Change Prices for This Product" next to the price type you want to change > Type in new prices > Click "Submit"
Honestly, I think this is a grossly excessive amount of steps we need to go through just to bulk edit prices, but it is what it is.