How do I know if other transactions such as billable expense or timesheets are linked to my invoice? I don't think other transactions are linked to my invoices. And, if the invoices don't have other transactions linked to it, then can I proceed with the instructions provided here ? 

When I tried to change the "customer name" to the project name (for it to be linked to the project), error is showing up stating "The transaction you are editing is linked to others. Are you sure you want to modify it?"