Your old iPhone is still usable. You paid for it, and it is your phone to do what you'd like with it.*
The SIM Card in your old phone will be deactivated. You can "unlock" your old phone and use it with any activated SIM card. I plan on taking mine with me to Thailand in July, get it unlocked, and use it with a local Thai SIM card.
Is there any way to reactivate that SIM card from the old phone for use on a different line? Or do you have to get a brand new SIM card if you want to use the old iPhone on a different line?
New. A dead SIM is dead forever.
New. A dead SIM is dead forever.