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@Patrick- I think the easiest way to reach (your goal - if I undestood well) should be to create another "new" EPMDocument (let say Creo one) and associate it to wtpart (with associeted link as "owner"). The "old" EMPDocument (ACAD model) can be associated to wtpart as "content" link type.
The result should be a wtpart that has both EPMDocuments associated.