What happened if I decide to keep my iPhone to trade in?

If you arranged a trade in and then changed your mind, you can just keep your phone. See the following from Apple Trade In - Apple

Can I cancel a trade-in?

If you haven’t dropped off or shipped your device, you can cancel your trade-in by simply keeping your device.

If you’ve already sent us your device, the trade-in can’t be cancelled. If you receive a revised trade-in value after the inspection of your device which is lower than your originally quoted value, you can then choose to reject it.