When going to the Payroll tab on an employee's profile you will always have the Change Basic Pay button, which allows you to change the employee's salary.
Next to this button you will sometimes see the Edit Current Pay button, which allows you to change their current salary line instead of creating a new one.
The Edit Current Pay button will only be available if the employee's latest salary hasn't yet been exported to Payroll. This is why you are still able to change it.
If the salary has been exported, the button will no longer be available and you will only see the Change Basic Pay one.