Can I change from one iphone to another?

yes, you'll get a new iPhone and SIM, and just reactivate it under your old telephone number

The sim cards are pretty sturdy and you may find that the sim card is undamaged. I'm not completely sure of this but you may be able to put your existing sim card into the new iPhone and sync it using iTunes. I could be wrong about this. Getting a new phone activated on your existing number will be the easy part, spending the money on the new phone is the worst part.