The iFixIt teardown of the mid 2013 MacBook Air shows the new style SSD drive. It's still on a dedicated daughter card with a metal edge slot like the previous generations. The connector isn't compatible with the old one.
That means that it's theoretically possible to upgrade the storage in the future but that only Apple has a compatible card to do it at this point. It's possible third-party manufacturers might try to make compatible replacements like they have done in the past.