Do I remove the SIM card before trading in an iPhone?

Yes.

Generally, the easiest way to activate the new phone is to use the SIM from the old phone. I've had the same SIM for about the last 4 iPhones.

But, if you don't chose to do that, it doesn't matter what you do with it. The SIM will be disconnected from your mobile number when the new one is activated. It will just get thrown away.

edited: totally misread the question

It wouldn't matter.

Cbeaux wrote:

Do I remove the SIM card before trading in an iPhone?

Yes, remove it and insert it into the new phone. This will transfer your carrier account to the new phone.