Jayme: I know your post is several weeks old, but I didn't see it till today and I'd like to respond. In many (but not all) cases, an "upgraded" unit is indeed absolutely identical to one that was manufactured as that particular model. Customer service folks at Audio Research (the king of the upgrade game), Mark Levinson and others will verify this and they'll tell you whether they have to make any compromises when upgrading a component.
Still, I think a prospective buyer should be told that the unit in question did not begin its life in its present form, but a lot of people have a mistaken notion that an upgraded unit is somehow not as good.
Still, I think a prospective buyer should be told that the unit in question did not begin its life in its present form, but a lot of people have a mistaken notion that an upgraded unit is somehow not as good.