How to change a non-billable project to billable

The client provided a listing of project types that should be non-billable. After all initial configurations are completed, Project Conversion took place. During Conversion validation, it was discovered that some projects that were converted using a non-billable project type should have been billable. This discovery took place after expenditure conversion. Some projects that were created with non-billable project types now need to be changed to billable. How can this be accomplished?

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