For anyone comming acros this thread with this issue:
The storage type ElementId is a storagetype that can be constructed via the Revit API.
id = ElementId(<int>)
It requires a integer value to cast into the ElementId Storagetype (it can be obtained by using the
.Id property on any element.
If you get this storage type in your Dynamo script, it means that you are at that point handeling the ElementID of the element you got the parameter from. This is the same ID as is shown in green behind the elementname in the dropdown menus. you can also use this ID to find the element in your Revit project.
An important thing to realize is that Revit handles almost everything as an element. Both the instances you place, as the familyTypes and the family to which the types belong (found as a familySymbole) are considered elements.
In this particular case the ElementID refers to a familyType. This ElementID can be set to the corresponding familyType ID of a different type of the same family. This way it should change the familyType of the family you’re handling.