If I select a new bid adjustment, which kind of bid adjustment will the tool override?

The tool will override any previous hourly bid adjustments that were there for the campaign in Google Ads, 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 Google Ads, we recommend making changes campaign by campaign, as then you will be able to see any existing bid adjustments the campaign has.

I enabled “hide campaigns that were modified in the last 14 days”, but I’m still seeing the modified campaigns.

The system stores the data from the moment you link your account to Optmyzr, and only then begins monitoring the changes. It can’t access Google Ads’ historical data, so the changes will only be recorded from the moment your account is linked to Optmyzr.

Only after this tracking begins, and when using "last 14 days" option, will the tool hide the campaigns for which bids were modified.

The tool isn't shrinking the list of campaigns where hourly bid adjustments were already applied.

Optmyzr stores historic bid adjustments which you apply to your campaigns from both Optmyzr as well as from the Google Ads interface. Because of this, it takes the system around 24 hours to access the historical performance data from Google Ads for the bid adjustments to be available in the Hour of the Week Bidder, so if the changes were applied to your account on the same day your account is linked to Optmyzr, then the system won't have that historical performance data available just yet.