I am surprised that someone like you are confused. Travel with both passports and they will then stamp you in according to your extension date in the new passport. I am assuming that you have a single re entry, therefore in this case it will be marked used. You then visit where you did your extension at your leisure to change stamp to new passport. If its a multi re entry just travel with 2 passports until your extension is due. Sounds like a non issue to me.