I don't think they ever called it Q100.5 on the air despite that logo. They should have, however. When they moved to 99.7, I was expecting them to do it, but they didn't. While it's easier for listeners to know they're on the right station, I suppose management felt the Q100 brand had too much equity to mess with.
I think they did the right thing for the long term by finally changing it...although I must admit it sounds strange after all this time as Q100.