Product: NetSuite 2020.1
When a user creates a new Price Level wherein the Update Existing Prices is checked and there are already transactions entered prior to the creation of Price Level
Updating the Markup/Discount % for a Price Level will not have any effect on already existing transactions using the same Price Level. The change is prospective.
The Update Existing Prices checkbox is only meant to update the Price Level as far as Item Records are concerned.