Avoiding duplicative CW and CT logs when registering an account to Control Tower

I want to launch Control Tower in a NEW management account, and then register the existing OU to the Control Tower.

This will mean that the accounts you want to enroll in AWS Control Tower will have to leave your current AWS Organization and if you have existing account-level CloudTrail, it would still be enabled and still work after leaving your current AWS Organization and joining the AWS Organization of the new management account.

There are separate Amazon CloudWatch Log Groups set up by AWS Control Tower which will not impact your existing CloudWatch configurations. You can review the mandatory controls from Control Tower

When you configure Control Tower, you will Optionally configure AWS CloudTrail. If you have chosen for AWS Control Tower to create your organizational-level trail or you create it yourself, then you will be subject to additional charges for Cloud Trail duplicate management events. You should just choose either organizational-level Cloudtrail or account-level CloudTrail.