Answered by Shaykh Faraz Rabbani
Situation A: A person is Shafi and does 1 rakat witrs for X number of years. This person becomes Hanafi and wants to make up missed prayers he has in the past. Do these witr prayers have to made up?
Situation B: A person is Shafi and doesn’t do any witr because its not wajib on them for X number of years. The person becomes Hanafi and wants to make up missed prayers he has in the past. Does he have to make up the witrs?
Answer:
Walaikum assalam,
There is a legal principle that, “An ijtihad is not invalidated by another.” [Kasani, Bada’i` al-Sana’i` 7.6; Abd al-Aziz al-Bukhari, Kashf al-Asrar, 3.74; and almost every major Hanafi fiqh work, with similar wordings]
From this principle, a derived ancillary principle is that, “Following a valid ijtihad in the past does not need to be rectified by one’s following different ijtihad later on.”
As such, if the person prayed 1-rakat witr as a Shafi`i, or even omitted praying it, no makeup would be necessary if one later became a Hanafi.
Of course, if one chooses to make it up, out of caution or taqwa, one is rewarded. It is a sunna to makeup missed witrs in the Shafi`i and Hanbali schools. [cf: Ibn Hajar, Tuhfa al-Muhtaj; Buhuti, Kashshaf al-Qina`]
And Allah alone gives success.
Walaikum assalam,
Faraz Rabbani.
BTW: Ahlan wa Sahlan