isn't it obvious?
You can not use Post-Processing vars in your model. But such necessity may arise when modifying the project.
GUI does not allow switching the var type.
You have to delete it from all the formulas, reload the project, delete the variable, then create it again of different type, and again type in every of countless formulas you have in the project.
Not very convenient.
I was thinking about accessing the property with VBS script, or directly editing the project XML file.
Not sure will it work, or not. Now I have different tasks to solve before trying that.