It's funny, because I had kinda the same experience (back when S1 started) - and then, sometime later, re-watched the pilot and was just totally thrown by how different the original Mitchell was. Like watching the original pilot of Buffy with a different Willow. (Or, like someone else playing Spike.)

*loves Mitchell*

(Also love Annie - liked pilot!Annie a lot, but I *adore* show!Annie. ♥)