Well, the change is still simply the difference. The accumulated real GDP can be different. But why would that mean that real GDP isn't properly defined. Your link to thermodynamic; in thermodynamics the values that are path dependent are as well properly defined. In the real world, the economy has taken one path.

And it is very clear, that Zimbabwe isn't the richest country in the world, despite for sure by far the largest nominal increase in GDP.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers