I just changed to a Sureflap (Tom cat poking head through previous cat flap). I've only just changed it, my cat was outside when I did and when he came back put his head in and pushed it open slightly but then retreated. (Already registered his microchip before fitting.)
He's in now as I opened to door to put the screwdrivers away in the shed. Now I had a bit of hassle screwing the screws in so I'm a tiny bit wound up so can appreciate I'm a little uptight.
But that normal isn't it, it's new he's cautious but he'll use it won't he?