If I switch SIM cards, what will happen?

Right now my phone's acting messed up and only receiving about 50% of my text messages. I've tried everything- restarting my phone, resetting the network, calling into my phone company, etc. I have an iPhone 4s. This is really irritating, I've been having on and off problems for the past month or so.

Rogers finally suggested for me to switch my SIM card, then call in and transfer my phone # onto the new SIM card. I already have a new empty SIM card but I just want to know ahead of time… What will happen when I switch it? Will my contacts, messages, and all that still be on my phone? The guy said that the SIM card only holds the phone # itself, and all my contacts/messages will still be on my phone. Is that true? I've tried researching online but can't seem to find anything useful. Does anyone know?

Yes that's true. I've got an unlocked iPhone 4 which I have used with 3 different service providers.