When change the login ID of an existing account to another, it shows has been saved but actually the new login ID never works. The login ID is still the previous one.
Below screenshots are taken using OOTB to replicated this issue.
Full Name and Login ID are 2 must fill fields.
Create a new enduser called "Eric".
Soft delete Eric. When try to create a new enduser called Eric. The below error pops out. This works correctly.
Create a new user called Eric2.
Modify Eric2's login ID to Eric and save. Console displays it has been changed to Eric in the below screenshot. But when reload this Enduser, the loginin ID is still Eric2.
The change does do work actually.
There is a enduser which login ID already exists in soft deleted records. So it can not be changed as desired.
Restore the soft deleted user or use SQL script to hard delete the user in DB. Then the login ID canbe changed to the desired one.
The best way to avoid this happen in future is before soft delete a user, add prefix to the primary key, such as XXXX_. If delete the user this way, this issue will not happen.