SUS - how to re-assign user id vendor to its company

Dear SAP experts,

We are in SRM 7.01 with ECC 6.05 deploying classic scenario and running MM-SUS. In our company, user id for vendor will be created in SUS following the vendor code in ECC. So if vendor code in ECC is 12345 then we will create user id 12345 assigned to this vendor company. This is to make it easier for us to manage the id.

However there is a mistake where someone is creating user id 12345 and assign it to vendor company 45678. When we want to change the assignment it is greyed out already. Is there a way to change the assignment?

Best regards,

John