In my concept of the future there will be no "Save", but instead any input/document will be passively cached locally in volatile storage, and archived incrementally on a non-volatile medium... without any active user input. In which case you'd only need an icon for "New", "Open" and "Delete".
If you're talking about representing a change of state (volatile to non-volatile), then borrow from Chemistry where a change from liquid to gas is represented by an arrow up, and a change from liquid to solid is represented by an arrow down. You could use the Unicode character U+21DF, a symbol that's already on every computer. Inversely you could use U+21DE to indicate "Load".