Since you have one of the Republic legacy phones on the old plan you will need to get a new SIM (and plan) when you get a new phone. The SIM in your Moto G needs to stay with that phone and should not be moved.
If you order your phone via Republic it will come with a SIM installed. If you go the BYOP way, you’ll need to order a SIM. (The Community can help you more with this if needed.)
You will need to upgrade to the My Choice plan. Your current plan is only available for the old legacy phones. It’s explained in the article I previously gave you.
One more thing to think about is your current phone has CDMA (Sprint) coverage. If you order that BYOP phone and order a SIM, it will have GSM (T-mobile) coverage. I’ll post a few link on how to order a SIM and how to get a CDMA SIM if needed:
I’m just bummed out that the unlimited data plan I have is attached to my dying phone. Built in microphone has gone KAPUT but when I use earbuds with mic it still works… soooo… likely going to wait to upgrade for a bit.