Replacing a Kosher Product with a More Stringent One

Is the obligation to buy a more stringent product by a third only applicable when I am in the store, or even if I have already purchased, must I buy another that is a third more expensive?

Hello, There is a disagreement whether the obligation applies only at the time of purchase, or even after one has purchased a mitzvah item, whether one is required to replace it. Practically, if it is possible to replace the product but it involves effort, or if it is possible to sell the product and buy another, essentially there is no need to do so, but there is an interest in replacing it. However, if it cannot be replaced, there is no obligation to buy a new one. However, if the stringency is not a matter of beauty but a matter of different opinions regarding the product's kashrut, even though it is essentially ruled kosher, it is advisable to replace it in any case.

This matter is disputed by Rabbeinu Chananel and Rashi (Bava Kama 9: and their opinions are disputed by the Beit Yosef Orach Chaim siman 656 and the Gra is brought in Shaar HaTzion siman 656 sk"2) Tosafot (Bava Kama 9:) Rabbeinu Tam (Sefer Mitzvot Katan mitzvah 44) and the Rosh (Sukkah chapter 3 siman 12). For practical halacha, see Shulchan Aruch (Orach Chaim siman 656 paragraph 1) Magen Avraham (siman 656 sk"4) and Mishnah Berurah (there sk"4) and Biur Halacha.