Odd is the most affected when attacked by the Spectre in "Spectromania" (the others get sick; he gets deaf) and the one who has the most difficulty using the Tower. The end of the episode reveals he is the one who lost the most source-code.
As much as the poor quality of XANA's attacks can be blamed on shoddy execution, he really can't do a lot of the more flashy attacks he used before because killing the heroes is not an option anymore.
As pointed by the actors themselves in an interview, Sissi did mention in an original series episode that she dreamed to be blonde. Assuming she just dyed her hair, the change then makes sense.
William is the first to got to Laura after the return to the past doesn't work on her. Well, the same shtick happened to him months ago.
A rather obvious one, but the first hint that the ninjas aren't Xana's mooks (as the Lyoko Warriors and the audience probably thought at first) is the fact that they lack the Eye of Xana.
Why does Odd have brown hair instead of having his trademark blonde hair from the cartoon? Well, beside the fact the actor himself wasn't blonde, it was shown in the previous series that both of Odd's parents were brunettes, which means he could be originally brunette but simply chose to dye his hair, which would also explain his purple streak. He might have simply decided to keep the purple streak but not dye the rest of his hair, lazy as he is.
