Not true. Everyone has their own opinion though. I live in Des Moines and I see no such thing. I know ISU fans like to claim this is happening, but the ratio of Hawk fans to Cyclone fans has not changed. I just flew to Tampa and back last week, and between my flights I encountered at least a dozen people wearing Hawkeye gear, and only saw one guy wearing Iowa State clothing. I know that's not a fair study for statistical reasons because it was only two flights from Des Moines to Tampa and back, but I was quite surprised.
I think the only thing that has changed is that Cyclone fans are a little bit more apt to attend their games and wear their gear. It started with some of the momentum Rhoads built in his first couple years (Okie State win was huge), and then right when his momentum started to fade Hoiberg began to turn their basketball program around. Because of these things they are more "pwoud" to display their colors, where as before it was just kind of embarrassing to admit you were a Cyclone. I honestly don't think the numbers themselves have changed much at all.
My buddy explained this simply.
"ISU fans are like the gays(NTTAWT,) there are no more of them than there were fifteen years ago, but now their proud of it and seems like their everywhere."