To get the most benefit from campaigns, and to be ready for the end of the migration period, make sure that you configure campaigns to target 100% of your account's traffic. You may start migrating with just a test campaign, and add more over time to complete the migration.
To see whether your account has traffic remaining that isn't targeted by campaigns, open your account in the Hotel Ads Center, and click the Dashboard tab.
This tab shows performance grouped by campaign, and includes a special row with the same name as your account. This row reflects performance for ads managed directly by your account or hotel groups (legacy bidding). If this row has performance metrics like impressions or clicks for the most recent period, then you may need to adjust your campaigns' targeting or create additional campaigns to fully cover the traffic your account targets.