If for a valid reason, as mentioned by other brothers, then it is permissible for him to change to the Shafi'i mazhab. I detect certains unhealthy tones of mazhab comparison here. Sorry if I sense wrong. IMHO all Imams of mazahibs were among the greatest of the Awliya. I am proud to follow Imam Nasir al-Sunnah Muhammad bin Idris al-Shafi'i but that doesn't mean I look with less respect towards Imams Abu Hanifah, Malik and Ahmad bin Hanbal, ridwanallahu alaihim. Just a thought that comes to my mind. No harm intended. Wallahu a'lamu bi al-sawab.