Must one make up the prayers that the congregant completed as Jumu'ah, thinking that whoever joins while the Imam is rising from the second rak'ah completes it as Jumu'ah, after he learned that it is obligatory to pray it as Dhuhr?

Scholars differed concerning one who caught less than one unit (rak'ah) of the Jumu'ah prayer. The majority opinion is that such a person should pray it as Dhuhr, four units, based on the hadith: "Whoever catches one rak'ah of Jumu'ah has caught the prayer." The Hanafis, however, are of the view that such a person completes it as Jumu'ah, regardless of how much of the prayer was caught with the Imam.

It is more cautious for someone who caught less than one rak'ah of Jumu'ah to repeat their prayer as Dhuhr. However, if one follows a weaker opinion after the act has occurred, which has been permitted by many scholars, then there is no blame upon you for emulating those who held the view of catching Jumu'ah in this situation.