The Faithful - S3-E4
Trivia: James and Winn find a meeting of Coville and his followers to be "a little cult-y." Melissa Benoist plays Rachel Koresh, the wife of cult leader David Koresh, in the miniseries Waco, a dramatization of the Waco siege - the 1993 standoff between the ATF and FBI and Koresh's Branch Davidians that lasted 51 days and resulted in 86 deaths.
Trivia: Erica Durance plays Melissa Benoist's mother in this episode. She played Lois Lane in the TV show Smallville, about Superman when he was young.
Trivia: The way Kara rejects the bullets from the machine gun is very similar to how Superman did in Superman Returns.
Answer: Another part of why the change was that it's cold in Canada in the winter and the old costume didn't protect her from it, whereas the new one is nice and warm.