Is it necessary to re-perform acts of worship that were carried out according to an old madhhab, after transitioning to another madhhab based on scholarly preference, an example of which is a pregnant woman who broke her fast and paid the fidyah, then it became clear to her that both qadaa’ (making up the fast) and fidyah are obligatory?

It is not obligatory to adhere to a specific madhhab (school of thought). A muqallid (follower of a madhhab) may switch from one madhhab to another under three conditions: that he does not combine between them in a way that contradicts consensus (ijma'), that he believes in the superiority of the one he is following, and that he does not selectively follow the dispensations (rukhs) of the madhahib. If someone switches from one madhhab to another after having followed the first madhhab, he is not required to re-perform the acts of worship he carried out according to the previous madhhab, because he fulfilled his obligation and discharged his responsibility. As for those capable of tarjih (weighing evidence and preferring one opinion over others), they are not permitted to engage in taqlid (following a madhhab without critical evaluation); rather, they act upon what they deem strongest based on the evidence. If she acted upon what she found to be preponderant regarding the non-obligation of making up a prayer, and then later found its obligation to be preponderant, she is not required to make up the prayer.