I actually think there have always been a fair number of "converts" in our support. United appeal to little kids who know nothing. It's that glitzy, tacky, Take That, follow the crowd thing that they have, or certainly had a few years ago.
Kids grow up and a fair number realise that City have more soul, more honesty, and are an awful lot more loveable than that PLC down the road.
It's almost the other way round, to a degree, I guess now. I've moved to Yorkshire and amongst the under 10s (my boys play junior football) there are loads of City shirts.