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As with the MacBook Pro, it's likely the 12-inch Retina MacBook Air will initially be sold alongside existing MacBook Air machines due to differences in price. Over time, the Retina MacBook Air may replace the existing MacBook Air, just as the Retina MacBook Pro replaced the standard MacBook Pro.
The Retina MacBook Pro never completed replaced the standard MacBook Pro. You can still order it. Granted, it hasn't been updated in almost 3 years...