Solution
To change a property node from Read to Write, you need to use the right-click shortcut menu to change the Read/Write operation of the property. Right-click on the property node and click "Change To Write" / "Change To Read".
(Alternatively, you can use "Change All To Write/Read" if you have multiple properties in one node.)
These steps are illustrated in the picture below:
A small direction arrow to the right of the property indicates a property you read. A small direction arrow to the left of the property indicates a property you write.