We can change this on the employee record but only until the user has posted transactions. If the user has posted any transactions then the only solution I know of is to inactivate the user and create a new employee record for them under the new Subsidiary.
The reason why the Subsidiary field is not editable in the Employee record is because there are existing transactions involving the employee. This is also documented on SuiteAnswers ID: 94702 Unable to Change Employee Subsidiary.
Enhancement 150756: One World > Multi Sub Entities> Ability to assign Employees to multiple subsidiaries.
If we created a new employee record, it will be as if its a new Employee, all history records financial and lead and opportunities will be related to the old record, this is rather confusing for work process. Unfortunately, it is a tedious process to transfer the records from the old employee record to the new one. And regarding the merging of employee records, this is still logged on
Enhancement 126644: Lists>Mass Update>Mass Duplicate Record Merge.<br>Include the ability to select for the Employee entity as well.
Enhancement 150756: One World > Multi Sub Entities> Ability to assign Employees to multiple subsidiaries