Silver eye's complicated, and I don't understand it all the way myself. There's a small sticker inside the eye, and it can come undone over time. It may never come undone, it may come undone all the way and turn the eye silver, or it may come undone just a little bit and make the eye appear to turn gray just at the 'spokes'. (Most of the AG dolls, but not all of them, have a spiral design to their eye. All of the blue eyed dolls do, and it's easiest to see on them, so if you've got a Kirsten around, go pick her up and give her a good look.)

The easiest answer is that, so far as we know, the doll's eyes didn't turn because anything the owner did. If you have an extra pair of eyes around, or if you have a doll that will sacrifice her eye(s), you can swap them out yourself, but I hear it's hard.