Thanks for your first post, @paul-eaglesoncle.
At the moment, you can only convert sub-customers to a project. While the ability to convert a project to a sub-customer isn't an option.
For now, you'd need to create a sub-customer and change the transactions in the Project to appear under the Sub-Customer profile.
For more information on using Projects in QuickBooks Online, check out: Set up and use the Projects feature.
Please know that you're always welcome here in the Community, @paul-eaglesoncle. I'd be around here to lend a helping hand.
