SIM card and a new phone upgrade

Tweet

So I switched to AT&T after being with Cellcom. I have the samsung mythic with unlimited data and 200 texts, with the 450 w/rollover minutes. Since I was on a network that was CDMA, I never really knew what a SIM card was. Now, my phone has a SIM card. My question: is ALL my information on my sim? Like, is my plan (unlimited data and 200 txt) on my SIM? I really like the samsung mythic, but lets say I find this really sweet deal on a smartphone (samsung mythic isn't considered a smartphone) could I get the smartphone, put my SIM in it...and STILL have unlimited data and 200 txts? The reason I ask is this, the unlimited data plan for a smartphone is $10 more (because it includes unlimited txt, not just 200). Samsung Mythic is considered just a 3G phone. Would I be able to upgrade and still keep my same plan? Heck, would AT&T even know? Any information on this topic would be GREATLY appreciated :) Thanks!