Crisis on Infinite Earths: Part One - S5-E9
Trivia: One question Marv Wolfman continues to get asked at fan conventions is why he killed Kara off in the "Crisis" comics. With so many characters established as having survived Krypton's destruction - including Clark, Kara, Krypto the Superdog, criminals banished to the Phantom Zone, and the bottled Kandor - his goal was to make Superman unique as the planet's sole survivor; killing Kara gave him the creative freedom to return Clark to his origin as the Last Son of Krypton.
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.