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.
- it's awfully and poorly believable but probable, Inukai created the Correctors just as part of Grosser's own actions to disobey its programming, programming that Inukai (and only him, well, according to himself) programmed to control Com.Net. the bear stealing the new Com.Com was due to Ai (the messenger) permission to be whenever it wishes (and needs) to be being shared, the bear itself stole it cause it can help the messenger to recover of the virus, it was totally unexpected for the teddy bear to being related to the Correctors. its still probable cause looks like Yui's father was on the Com.Net's list of founders, if Grosser was created 10 years ago with Com.Net (2010??? :o) then Grosser's Stalking With A Crush can be related to one part of its programming being influenced by Yui, but thats too farfetched.
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.
- She was searching for that "girl with the bear", not the virus, maybe she was just following the trace with some sort of detector, but not every day and not with her suit loaded all the time, she had another one for her own private virtual laboratory when she passed out her time on Com.Net xD.
Com.Net is Haru..., I mean, Second Life
yes, it's so similar that can't be coincidence, maybe all will be connected to Linden Labs on 2020 or can be some sort of recent and better competition but the similarities are stunning: people can teleport, fly or being forced to walk, can dress whenever they want with its own avatar, can change its avatar to give habilities they don't have on Real Life, there are spaces where anything can be programmed, from buildings to AI's and can be interactive with them, can get lost on that world... or maybe everyone has its own servers with its own versions but all is still connected to one central location explaining the infections and deletings of all Com.Net at the same time that started as localized infections on one server.
Humans Corrector Suit theme: fairytale key characters
its 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