I have two phones on my account. I bought an unlocked moto z and have successfully activated it on Republic replacing one of the phones on the account. I know want to give the replaced phone to the holder of the second account and maintain the number on the second account moving it to the phone I replaced with the moto z. The SIM on this old phone was once on Republic. I have the #s off the SIM.
You don’t need to do anything with the SIM on the Moto G (3rd Gen) you replaced. Leave it in the phone.
Factory reset the Moto G (3rd Gen) and then activate it in the second account. If you activate it in the second account as an additional phone it will get a new number. If you activate it as a replacement for a phone in that account it will acquire that phone’s number.