You might need a micro SIM card or a nano SIM card with some smartphones. Those two models are typically smaller than the classic SIM card.
Does your current SIM card not fit in your new handset?
You can easily buy a multi SIM card online. A multi-SIM card consists of three pieces: a standard SIM card, a micro SIM card and a nano SIM card. You simply push the smaller card out of the larger one.
Your current SIM card remains active until you have activated your new SIM card online.