Q: How to people at genius bars switch sim cards?

ssrossiter wrote:

So basically if I haven't backed up my phone, I will lose everything?

Maybe. If the info is sync'd to your computer it will be fine

But see this -> Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support

tell me what I keep on the sim card

Nothing.

ssrossiter wrote:

 

So basically if I haven't backed up my phone, I will lose everything?

If I am wrong tell me what I keep on the sim card

That is correct. You will lose everything, unless it is content that you have synced to supported programs with the phone or cloud service, such as contacts, or calendar.

Again, there is nothing on the SIM card from your phone, nothing. As indicated earlier, the SIM has nothing but the account information from your carrier.

ssrossiter wrote:

 

Yeah Im not worrying about that.

People kept telling me that they run the sim card through a machine to download all that cards data to put it on another sim card

People told you wrong....