It is a micro Sim. In store they usually insist on new cards, maybe to justify the "upgrade" fee. Think you may be able to use one from an iPhone and just swap them, but you still need to make sure your account is "provisioned" for LTE.
If you get one delivered home, it will come with the new sim. You have to call and activate it by phone.
That's where you would be able to use an existing sim card...if you have MicroSIM or willing to cut your own.
Like this tool:
http://www.amazon.com/gp/aw/d/B003U...ag=hawk-future-20&ascsubtag=UUacUvbUpU1828299
Sent from my Samsung Galaxy S2 Skyrocket SGH-i727 on ATT LTE