At present, users can change "beam, spring, mass, 2d element, link and cable" properties between stages. Introducing the option to toggle material properties between stages would be useful when you consider the following;
In order to change material properties between stages, say for changing concrete from short term to long term properties, it is necessary to generate duplicate beam section entries for all elements, assign one set short term properties and long term properties to the other. Then a separate entry for each beam section property needs to be defined in the analysis stage properties window. If you had 100 original section definitions, you would need 100 entries in the analysis stage properties window. If we were able to change mass properties between stages we would not need to generate duplicate beam section properties and the multiple analysis stage properties entries would be reduced to a single entry. This would be quicker to define, result in less data and would be easier to check that stage definition is correct.