The reason your cant change the unit label at runtime is because unit labels change the data type of the wire (notice the "S" in brackets at the end).
That said, what you want can be achieved with the display format.
Enable unit label and specify the unit "S"
Change the display format to SI notation and the number of digits to 3
Now it will automatically add the prefix according to your value. For example, 10000 S will turn into 10 kS.