there is no believable theory yet, so I have mine, it weights the same as the rest.

My theory goes like this.
1 When you are looking at something close, the focal points move to behind the retina
2 To correct that, the lens has to become thicker
3 this signal feedback cause the eye to grow longer, so that the lens doesn't have to be thick

this is how myopia develops.

However, if you close your eyes, and press on the lens, the lens becomes thinner. This gives a negative signal that develops myopia.

No you can't change the shape of the eyeball, but you can certainly affect its grows. Why? myopia itself is a proof.

We are not talking about change the shape of an eyeball, but its growth. Remember even watermelon can become cubic.