If you convert a sub-customer to a project, will it convert all costs and revenue attached to that sub-customer or just for the current year?

Yes, Eduardo. Converting a sub-customer to a project in QuickBooks Online (QBO) will also carry over the costs and revenue. I will offer additional details to help clarify the process and address your question.
 

When converting the sub-customer to a project, all transactions related to that sub-customer, including costs and revenue from the current or previous years, will be transferred to the project. Please note that during this process, there is no option to specify a start or end date for when you want these changes to take effect in the system. However, you can make changes after performing the conversion in QBO.
 

I also recommend checking this article to learn more about the process: Change sub-customers into projects QuickBooks Online.

On the other hand, I recommend viewing Step 5 in this article to learn more on how you can maximize tracking the projects with a use of reports: Track hourly labour costs and profitability by project in QuickBooks Online.
 

Do you also need help customizing reports to enhance their detail in QuickBooks, or do you have other questions about the program? I am here to help. Please click the Reply button below, and I will promptly address your concerns. Stay safe.