Thank you for reaching out to the Community, @Ruben.
I can help share information about what happens to the data during the process in QuickBooks Online. When converting sub-customer to a project, any information from their details such as addresses, payment and billing, and attachments won't carry over.
Right now, it's only possible to merge existing transactions into a project. I can see how beneficial being able to revert and retrieve this type of information, but it's still unavailable.
I appreciate every input on this feature, and I'll make sure to let our product engineers know about your feedback. We always improve the features in QuickBooks Online, and this preference might be added in the future.
As we assess this, I suggest you visit our QuickBooks Blog to stay updated with all the changes that are being made.
Additionally, I recommend the following resource, which contains detailed steps and screenshots that will help a user learn more about what happens to the data: Change sub-customers into Projects.
If you have additional questions or concerns while working in QBO, leave a comment below. I’ll be happy to help you out.