The tool will override any previous hourly bid adjustments that were there for the campaign in AdWords, and they will be replaced with the changes you're applying. When our system makes changes to hourly bid adjustments through the Hour of the Week optimization, it has to rewrite the all the hourly bid adjustments and replace them with what you've set, which is why you see this message. It will not change any other type of bid adjustment.
If you already have hourly bid adjustments in AdWords, we recommend making changes campaign by campaign, as then you will be able to see any existing bid adjustments the campaign has.
Our award-winning customer care team is here for you.