Can I transfer data to new phone before moving SIM card over?

But to unlock the old phone, don't I need the SIM? Would I move it back to the old phone for unlocking? I have heard that you can do everything on the new phone, then at the end move the SIM over. You indicate I have to do that before I start. That is why I'm so confused...

You shouldn’t need the SIM to unlock the phone. A carrier unlock is done on Apple’s servers. Your carrier sends an unlock request to Apple, and Apple updates your phone’s carrier lock status.

Very interesting.

Also, when I erase the phone (as part of the unlocking process), can I just power it down once I see the Hello screen? Any suggestions?

Just a quick update. In the last 2 days, I upgraded my wife's iPhone 6 to a new iPhone 11 and I upgraded my iPhone 8 to a new iPhone 12 mini. Since my wife had a 6, I could not do phone to phone transfer. I DID NOT move the SIM to the new phones in both cases. Wifes, I restored from iCloud backup. After complete, I transferred the SIM from the 6 to the 11. For my transfer, I used phone to phone. AT&T told me to keep the SIM that came in the new phone and activate the new SIM. I asked if I could just use the old SIM, they told me NO. I didn't really believe her, but activating the new SIM was easy, so I went along with them.

The reason you shouldn’t move a SIM to a 12 is that it needs a 5G SIM to work correctly.

Well, that seems reasonable. Although a friend of mine upgraded to a 12 Pro and used his old 7 SIM.