Hi Keith,
I do not know of a way to change an object’s ID,
however the method
Rhino.ReplaceGeometry (strTarget, strSource [, blnDelete])
will allow for you to replace the geometry of an object.
This is useful when you for instance want to change the curves used for History,
without breaking the history.
What do you want to to that requires the change of an object’s ID?
-Willem