Yui was intended to be one of the correctors all the time.The difference is that she'd be only in the reinforcement team. Before you throw rocks at this poor troper, let's get it explained. Haruna was initially chosen by Inukai as the main corrector, and stuff, but IR went to Yui's computer instead, and was more surprised with her lack of computer knowledge than when he discovered that she was the wrong girl all the time. Also, IR couldn't convert Yui's outfit from a prize her father did so easily, if it didn't have a base in first place, which would mean that Mr. Kasuga is Obfuscating Stupidity and knew that his daughter is going to be one of the correctors as well, way before Inukai fell into a coma. Also, if you notice, at least in the anime, the three human correctors discover that they had a relative that participated in the Com.Net's creation (Yui's dad, Prof. Inukai, who's Haruna's uncle, and both Ai's parents). The fact that Ai's suit fit her could be another indication that it wasn't an accident, if you remember who stole the revamped com.com (I-chan's bear; both were programmed by Ai's dad, meaning that the Corrector project was planned all along). It might not make sense, but once you think about how many contrived coincidences are out there, you can't think Kia Asamiya wasn't thinking about that; the whole conspiracy plot would be in a third season, but since the manga had a so-so popularity, it didn't happen.
Ai wasn't really that quick, she was just as much Crazy-Prepared as the com.net team in the previous theory.She probably developed a quicker virus detector, and/or just waited at com.net every single day, after school, with her outfit already loaded.
Humans Corrector Suit theme: fairytale key charactersits just a guessing beyond "Yui is a Corrector Fairy, Haruna is an angel that can be a demon too and AI looks like a damsel", but I will like to know what do you think about the Human Corrector Suits theme, maybe there will be witches or elfs or even a smurfette :P