I had a Google Cloud project which I used with a "private" account user@mycompany.com. I wanted to allow my colleagues to the project, too, so I created an organiatzion mycompany.com for Google Identiy.
I wanted to re-use user@mycompany.com and had to delete it because it was a private account and thus blocked for Google Identity as far as I understand. In order to delete it, I switched ownership of the CCP project over to another private account other@mycompany.com.
Then I created the organization mycompany.com and moved the GCP project under its root. Everything runs smoothly, except I can no longer access the billing account which is still associated with the GCP project. I need access to it to download the invoice for the accounting department in my company.
I could replace the account with a new one and it'd be nice if it was configured in the organization rather than associated with a specific account. However, the invoice would probably lost and the accounting team would be in trouble as would I.
I can't use the GCP billing chat support because the first message I get is ~"you're not the admin of the project's billing account" without further options.
The GCP is still using the billing account and the ID is shown.
TL;DR: You can create a ticket to the GCP Account and Resource Recovery Request team by following this link
Besides chat, Google offers alternative options to contact support in case this escenario happen to you again. There is a Support Contact Troubleshooter page that will ask you some questions to define the best path of actions based on your escenario.
I followed the Support Contact Troubleshooter to find out that you could contact the GCP Account and Resource Recovery Request to file a ticket with Support.
The form can be used only under the following conditions:
You no longer have access to a GCP Project or Billing Account which you previously had access to. You must be able to identify the resource, to which you are trying to regain access.
There are no other resource administrators or Organization administrators who have access to the Project or Billing Account in question. (If there are, you must contact these individuals).
Your account is in good standing and has not been suspended.
The form will ask for your name, email address, Billing Account ID or Project ID and a brief description of how or why you have lost access.