To add they may be able to simswap for you instore which moves your mobile number and tariff to the new nano sim. Take some ID just in case your asked for proof of your identity.
A sim swap can take a few minutes to approx 24 hours to complete. Once service has stopped on your original sim this indicates the swap has started and then approx every 20 minutes or so turn the iPhone off and on or airplane mode On and Off to help the re connection of the nano sim to the network.
Current Phone >
Samsung Galaxy s²⁴ Ultra 512gb.