A project CREATOR can transfer the ownership of a project to one of its MANAGER(s) and become a MANAGER himself.
The newly assigned CREATOR can then remove the old one (now a MANAGER) at any time.
Go to the project details page and click on "Manage Users":
If you have a CREATOR role, click on the "TRANSFER OWNERSHIP" button:
Select a MANAGER from the list and click on "CONFIRM":
You are now a MANAGER.