Tracfone question regarding phone upgrading.?

[Olivia is due in Dec]

Hello, my niece has a tracfone. She would really like a new one and since I have her name for christmas I was thinking on looking at what they have out. I was just wondering if they are like any other cell phone and can still be used as long as it in the same company. Im not sure though because of the air time. If I would get her a new one would she lose all the mins she still has on the old one or would everything transfer over with the sim card. Tracfone's have sim cards right?