Cannot Change String parameter in Microflow

It seems not to be possible to change a String parameter via Change Variable or Change Object. Why is that?
1 answers

If you want to change the value, then only possibility would be to create a variable first using the value of your parameter. And then use that variable further in the microflow. 
However you can change object of an entity passed as parameter. The above limitation is only for primitive data types
For objects, please see entity access in domain model. I just tested it, I was able to change when you have Read,Write access