It appears, in spite of the photos not being real detail-defining, that the upper bird is a cockerel. It didn't "change", but has begun to mature due to hormones.
Note the more vivid colored feathers, and the saddle feathers appear more like a cockerel than a pullet.
It's hard to tell because it's blurry, but the tail feathers on the upper one look like they are hanging down at the very ends. If you see longer tail feathers beginning to trail downward, you definitely have a cockerel and not a pullet.