How do I transfer my SIM card to a new iPhone?

dwykerr wrote:

When do you change sim from old phone to new phone

That varies.

The migration tools will inform when and how that happens, if the tool and the carrier doesn’t move the cellular plan to the new device automatically. And if you want to move the plan.

Different carriers and different models can involve different migrations, such as migrating from a SIM model to an eSIM model.

And a migration doesn’t have to move the plan, if the user doesn’t want to relocate the plan.

Here is some related reading: Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

You can do it either before or after setup. When you first setup your new iPhone, you will go through a set of steps, which one includes activating your SIM or eSIM. If you have already setup your iPhone, you can go to Settings > Cellular and set up your SIM or eSIM there.