In VASSAL terminology this works the other way around–see the “Levels follow expression value” portion of the Layer documentation. This sounds like a straightforward application of that technique. A changing value belongs in a Dynamic Property rather than a Marker. The user is presented with commands in the Dynamic Property to set it to different values, and the Layer automatically tracks that property to know which image to display.
A basic hypothetical usage: you have a two-sided counter, one side is “full health” and the other is "damaged’. You choose to create a Dynamic Property called Health where a value of 1 represents full health and a value of 2 represents damaged. You create two commands inside your Dynamic Property that get exposed to the user–one sets the value to 1 and is called “Restore to Full Health”. The other command sets the value to 2 and is called “Damage”. Create a Layer trait called Status (or something similar) and tell it to track the value of the property Health. Set images for each level so the correct image gets shown when Status = 1 and when Status = 2.
