Your ATT SIM card can move over as needed, immediately.
Hardware wise, I agree- the Z10 is pretty cool ;). I tested one out a few weeks ago in an ATT store. Good quality rig. In the long term though (read, over a two year contract when you get time to play with your device thoroughly), you might want to put yourself in a better position app-wise. Android is the horse to beat vs BB OS. And the HTC One is impressive. Both BB & Microsoft are behind the curve- arguably, too far behind when you look at what you get inside the iOS & Android ecosystem.
I have a question that's similar to the recent unlocked phone thread, except I'm wondering specifically about whether I can move my sim card from an HTC One to BlackBerry z10. My company offers a limited number of handsets and I have the HTC One right now through them (granted, a pretty cool phone) but I really like the z10, so I'd like to be able to drop my HTC One's SIM card into the z10 and at least try one out for a few weeks (probably buy one on eBay). Our carrier is AT&T.