Pope, according to [Etymonline][1] is from: 

> - Old English papa (9c.), from Church Latin papa "bishop, pope" (in classical Latin, "tutor"), from Greek papas "patriarch, bishop," originally "father."

> -  Papal, papacy, later acquisitions in English, preserve the original vowel.

Given the Greek/Latin original term which was adopted also by Old English, how and when did the vowel change come about? 


  [1]: https://www.etymonline.com/word/pope