You encounter an issue that cannot change the login email in new Sophos Central account because the email address that you are trying to use is already associated with an existing Sophos Central account.
You will see this error while trying to change user role in Role Management.
To resolve this issue we have two ways:
1. Raise another account to Super Admin role
I want to reuse firstname.lastname@example.org in new Sophos Central that already associated in other Sophos Central called old one.
You will see Tom in Super Admin role
Go to Global Settings > Role Management > Edit > Super Admin, check the user wish to unassigned.
The last step is going to People from the left panel, look for the user john that is linked to the email address that you need to reuse and delete it.
If the above does not fix your problem, please refer to the second solution.
2. Send ticket request to Sophos Support for removing associated account.
Log in to the Sophos Central Admin using the email address you want to reuse.
Click on the login name from the top-right corner of the page > Account Details > Sophos Support.
Ensure that the Enable Partner Assistance is enabled.
Contact your Sophos Partner to request assistance in removing the administrator associated with the email address you wish to reuse.
Hope this post will help you to solve your problem.