Not if you already have the sims. I did go to the store because I was petrified of doing something wrong, but the guy there just did exactly what I would have done at home by switching the sims. I had already activated it at home using the sim from the line getting the upgrade, and he just took that out, put it back in the old phone, and put the unlimited line's sim into the GS4.
But if you are using a dummy phone in the equation you may need to go to the store.