Dave
This is going to replace an existing iphone, not a new line. What should I do, take out my sim from my existing iPhone and use that one or just use the one that's in the phone? I assume if I use the one in the phone the older Sim will become inactive and NO extra fees?
CC
As we covered above, reuse your existing nano-SIM and there is no charge! There is no magic or special feature nano-SIM. Over the years, SIMs do change and need to be replace to allow access to Carrier specific features. VoLTE / HD Voice is one example where a new "SIM" was/is required. You needed/need a SIM with 26 or higher in positions 9-10 in the ICCID from the front of the SIM card. You can check this without removing your nano-SIM to check. On your iPhone, Settings, Tap General, Tap About, scroll down to the ICCID line and check your ICCID. For reference the current Version should be 27 but again 26 or higher is good.
The normal activation process for any Carrier iPhone X in the USA, when the device arrives to you, it is ready to activate using the new nano-SIM by default. The activation process will activate the new nano-SIM and deactivate the old nano-SIM during the initial iPhone X set up.
The only way that this would not happen would be if you were able purchase an AT&T iPhone X from Apple as a "Device Only" purchase. This means that you provided "NO' information like your existing phone number before the iPhone X is scanned out during the purchase process. The normal magic of paring any AT&T iPhone X to your account is done by a simple Bar Code scan by AT&T or Apple. The information is on the phone's box that is needed to make for a quick and hopefully seamless activation.
Another benefit of reusing your existing SIM is that you don't have to wait on the overloaded carrier network to enable the "New" SIM on Launch Day.
Note: No account information, no Upgrade or Activation fee!
Note: Some AT&T accounts offer free Activations & Upgrades.
Dave