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Tatal agree. The most important action to do is redirect your old domain to your new one, and create new and fresh content for the new one.

So basically domain B now has domain A content etc without the iframe? And you are asking if you move it back to the way it was with a 301 redirect from domain B to domain A would you get your rankings back?. If domain A was set out the exact same way as it was originally. in time theoretically! yes! Kind Regards Neil