What happens if you take out your SIM card and put it in another phone?
You can take the SIM card out, put it into another phone, and if someone calls your number, the new phone will ring.
You can also put a different SIM card in your unlocked phone, and your phone will then work with whatever phone number and account is linked to that card.
Can I buy a new phone and just switch the SIM card?
With unlocked phones, you can always switch your phone service between different phones as easily as popping out the SIM card and moving it. You can also use different phone service plans in a single unlocked phone, say one for work and one for personal, by switching the SIMs.
Can I use my old SIM card in my new iPhone?
You can do either but is normally best to use the new SIM card that comes with the new iPhone. You will need to activate that new SIM card. Once you activate the new sim, the old sim will be deactivated.
What should I do with my old SIM card?
Destroy the Card
A cell phone or recycling center might be able to help but personally watching the card being destroyed is the only sure bet for protection. Smash it then ask them to incinerate the card or smash it then throw it away in your trash.