Hum, no answer about this recurrent problem ? I forgot to precise the soft : ArcEditor 9.3.1
I can ask the question in another way : Is it possible to ask toolboxes like merge, make feature layer, delete... to update their internal cache according to the static input structures if they change ?
Today, the only way that I found, if I modify the structure of an input shapefile (ie add field), is to delete the modelbuilder object (shapefile variable or toolbox like merge... ) and recreate it, or to edit the shapefile object to reselect the same shapefile, to force the update. Im my opinion, unless I didn't understand something, this way is not up to such a powerful and expensive soft like ArcEditor.