Hi Nebulous,
THAT is a feature that many of us Visio dev-types have long wished for. I think that updating shapes is a very difficult problem to tackle in a generic way, and that is one of the reasons there has never really been a tool created for it.
My
guess is that the add-on you found in the Tools > Add-ons > Visio Extras menu is used by Microsoft/Visio to update some of their in-box custom solutions, like the Org-Chart, P&ID solution, and UML. I have noticed, that when opening older diagrams of these types that some sort of "update" progress bar will spring into action.
Perhaps one of the Microsofties will comment on this post, or perhaps some of the other Visxperts out there have talked to our friends in Redmond and gotten the complete scoop...
Thanks for the GREAT question, sorry I don't have the magic that you need
