If you change advertiser spend options, all advertiser spend reporting reflects your latest settings, including reports on previous days' numbers. This lets you adjust the margin or markup you are taking retroactively.
For example, if you are achieving an advertiser spend eCPC of $2.00 but the client has a goal of $1.75, you might make the business decision to shave your margin to allow the client to achieve the goal. Your reporting would reflect this once you've adjusted the advertiser spend. No matter how far into the flight dates you are when you make the change, the Daily Reporting page immediately shows the new margin in the Adv. Spend column for all days in the flight.