The elements IDs can change but the GUIDs are stable. There is a node called Revit ID in the Hot Gear package that will give you these GUIDs.
For more explanation, please see on Revit forums:
http://forums.autodesk.com/t5/revit-api-forum/unique-id-vs-elementid-which-identifier-to-stored-in-the/td-p/4962636
And also, Id to Element from Archi-lab Grimshaw accepts GUIDs
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