Popes don't change their names, they adopt an official style that happens to include a name. Joseph Ratzinger, styled "Pope Benedict XVI". Shouldn't appear as Benedict Ratzinger or Pope Joseph.
John Stewart, styled "King Robert III of Scots". James Charles Stuart, "King James VI of Scotland", "King James I of England". Albert Windsor, formerly Saxe-Coburg-Gotha, "King George VI". Never George VI Saxe-Coburg-Gotha.
As a matter of practicality, it works best not to mix and match. Put the official style all in one piece in the nickname field and keep it out of the other boxes.