Given you have a worked with dragging in Hype before the JS needed to add and remove a class can be found here … basically all you need is to do the following commands:
element.classList.add("draggable");
element.classList.remove("dropped");
and vice versa …
element.classList.remove("draggable");
element.classList.add("dropped");
Update: Just saw that @JimScott posted something 
along these line! Two people same thought…