I don't think that a chimera would undergo a drastic change like that... if it were an amel/snow chimera, some areas (which originated from the amel cell lines) would be amel, the others (which originated from the snow cell lines) would be snow. But in order for that change to happen, the actual cell lines would need to change as it grew, but that should mangle the pattern in the process, too.
It could be genetic, it could be a disease, or a lot of things. :shrugs:
It doesn't look like a piebald to me, pieds don't start out one color and turn into another. They start out with white patches that stay white and patterned patches that stay patterned.
Calico is such a wildcard of a name that it could mean anything you want it to mean.
The legendary calico in corns was always accompanied by the formation of blisters where the color disappeared, so I doubt this is
that calico.
As always, there's only one way to find out if it's genetic.