Arduino Uno starter's kit and project
Each of these chips is programmed with a bootloader. The bootloader allows you to program your arduinos. This link says that the Optiboot bootloader is the one thats used in the UNO. In which case, both chips are the same and can be swapped.

However, if you are new and learning, I wouldn't swap them. As Jey was saying, there is the potential to damage the chip by removing it, static discharge etc...