Please, check a couple of options for updating the text of a particular shape and then redraw the shape with the new value - http://dojo.telerik.com/OjobUb.
First option is to change the individual dataItem of the shape, then use the redrawVisual method to apply the change to the visual. This method is a simplification of the the approach you proposed in your post. The drawback here is that the dataSource is out of sync with the shape.
The second option is to change the dataSource - will it be through a call to the server or update the data on the client-side. Then call the refresh method of the diagram. The shapes get the updated data from the dataSource and get recreated.
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