How to transfer the candidate account linkage from one user account to another user account

Content (please ensure you mask any confidential information):

Customer found that there are two user accounts for one employee in Taleo (Account A is created by Taleo with SSOID started with "tr", and account B is imported by HRMS through TCC).

Now we have checked that the candidate profile is linked with account A, but the employee use account B for his daily operation.

I would like to know how to transfer the candidate profile linkage from account A to account B. I have tried to check "Ask if employee has a candidate account at next login" in account B and enter the candidate account after login, but the action fail and return error message "The candidate information you provided has already been used to fill another employee profile. Please contact your system administrator.".