the Brady stove
Having just binged on the entire series of The Brady Bunch on DVD, I can attest that the yellow gas cooktop was only in the 5th (final) season. The oven got changed out that same season as well. I don't know why except to speculate that the stainless Thermador set up, while upmarket, might have not read "70s Fabulous" by the 74-75 season. Particularly in the case of the oven, which was replaced with a full-glass door style as opposed to the older style window on the Thermador. (Does anyone know what brand that newer oven was?)
I don't know if there was ever a clear enough shot of the gas cooktop to be able to tell the brand, but I've learned never to underestimate some of the members on here, either!
The avocado GE Americana side by side was still current enough looking to pass (if modernizing was the actual reason). Who knows, there could have been some sort of incident that happened during the summer hiatus and that necessitated repairing/rebuilding a portion of the set. We know that's what happened on Bewitched--a fire destroyed many of the sets, and that's how Samantha got her new kitchen.