I have an older tallit gadol which one of the kanfot got caught in a seat and is pasul. I purchased a new set of tzitzit for it, but now have a problem. I understand (possibly incorrectly) that I should not remove the other 3 which are still kosher. However, the tallit will look a bit odd, as the new tzitzit which I purchased are both longer, thicker, and far less worn than the remaining ones. (They’re kosher, but the tallit is about 15 years old). Can I replace the entire set?

There is no halahic issue with having tzitziot of different ages, even if one can see slight shade differences. Alternatively, you are also permitted to remove the other 3 tzitziot and replace them with new ones. Remember to say 'lshem mitsvat tzitzit' before and during the process. If the actually knaf (corner) of the beged (garment) is torn then your entire talit is pasul, despite what tzitziot you use. Rabbi Binyamin Bemberger