Amex used to allow you to product change among any of the Blues, although I'm not sure if they still do. Some have had success doing Amex product changes right away, without waiting at all.
I was just approved for an Amex blue last week and now I'm thinking I might want to change to the Blue Cash or Blue Sky Miles card. Anyone have an opinion? Do you know how long I would have to wait to ask for a product change?