First, check the mill date on the new bag of feed. The nutrients oxidize quickly in grains quickly after they've been milled. So does the vitamin amendment that is added to the feed. So, feed that is past 6 weeks, is feed that is well on it's way to becoming rancid. It's a good idea to check the mill date before paying for any bag of feed. It's my goal to purchase fresh, and have it used up before it is 6 weeks old. I'll walk out of a store before buying old feed.
Second, your flock is snubbing the layer b/c the protein content is lower than that of their starter/grower. IME, they will always choose the higher protein feed. And as other posters have stated, they hate change. If your girls have access to free range, they will often fill up on what they find on range, and use the formulated feed to "fill the hole" if range is found lacking.