Question, vaguely related to the Nexus 5...

Vixy just got a Nexus 5 and is very excited about it. It came fully unlocked with no SIM card.

She was expecting that she would simply take the T-Mobile SIM out of her current Samsung Galaxy S phone, and plug it into the Nexus 5. Alas, the SIM cards are different form factors.

We are planning a jaunt to the T-mobile kiosk at the mall today to get the SIMs rejiggered for her new phone.

My question is: Never having done this particular trick before, how likely are we to be successful when we talk to the drones at the booth? Is there anything in particular we need to watch out for? Are there special magic words I can say which will get them to successfully complete our task without staring at us blankly?