I can share some information about updating account information, @Remi B.
You can update the email address associated with the account from Intuit Account Manager. However, it will also apply to all your accounts under that email address.
Here's more information on how to change your email, user ID, or password for QuickBooks Online. Also, here's a guide on how to manage your data securely using Your Account Manager.
To help you in updating the email address associated to this account without affecting the other company, I suggest reaching out to our Customer Support. They have the tools to update this information in a secure environment.
You can reach our Customer Support for QuickBooks Online (QBO) by going to the Help icon at the top right of the account. Follow the steps below:
Here's the contact the QuickBooks Online Customer Support team article for more information.
If you have further questions with updating the account information, let me know in the Reply section below. I'm always glad to help. Have a good day.