>ERWin has this capability. ERWin can reverse-engineer an existing database into it's model. Afterwards you can have it compare the model against the database - or another database. This way, you can make structure changes to the database itself or to the model.
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>There is another ER modeling tool that has the same functionality, I don't remember which one. It was pretty recent, within a couple of weeks. Try using the search function and looking for ERWin.
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I think I mentioned EasyER/EasyObject - http://www.grey-matter.com/gmWEB/items/00012313.HTM