Transferring the project ownership of a project is possible both ways; someone with Admin status requests the project ownership or some project owner invites a project ember to become the new project owner.
This is done by accessing the Request ownership / Transfer ownership option under SETTINGS > PROJECT DETAILS. It will trigger a request for confirmation to the other party, who can accept or decline the petition.
Since the difference between Project Owner and Admin status is just formal (both roles can do almost everything), you don’t need to transfer a project for any practical reasons. This action is usually linked to a project owner leaving the project or the company in which such project is managed. On tree.taiga.io, it is relevant for the billing information, since only project owners are considered potentially billable (depending on their account type).