Best Practices For SIM Cards When Upgrading iPhone 6 To iPhone SE (Second Gen)

New iPhone SE arrived today directly from Apple, with a new Nano-SIM card from my carrier, AT&T, pre-installed, which was a surprise, as I had expected to transfer my old Nano-SIM card from AT&T (from my iPhone 6) into the brand new SE but now that might not be necessary.

Is there a best practice for this particular scenario?

Can anyone share their own experiences specific to AT&T, SIM cards and upgrading to new iPhones?

Would like to know if there's any downsides to using the old SIM card or if one should stick to the card that comes pre-installed on the SE.

As it is, all data, apps, and content on the old phone has been seamlessly migrated over to the new phone.

Thanks.