ما هو رأي ابن تيمية في الرجوع عن التقليد بعد العمل بالفتوى؟

It is permissible for a muqallid (follower of a madhhab) to transfer from one madhhab to another, unless their intention is manipulation and seeking out dispensations (talfīq and tatabbu‘ al-rukhas). This is what Shaykh al-Islam Ibn Taymiyyah and the majority of Muslim scholars have stated. This transfer does not invalidate the acts of worship that were performed based on the previous madhhab, even if they would be invalid in the new madhhab. Scholars have explicitly stated the permissibility of taqlid after an action has been performed, and that if someone prayed believing their prayer to be valid according to their madhhab, and then it became clear that it was invalid in that madhhab but valid in another, they may follow the other madhhab, and that prayer is sufficient for them.