Just wondering.
According to Bulbapedia, if you gain this ability from a Castform, it won't do anything. If you are Castform and you lose the ability, you'll be stuck in whatever weather form you're in currently.
"This Ability is made specifically for Castform, so if other Pokémon obtains this Ability, it will have no effect. Conversely, if Castform loses Forecast, it will be stuck in whatever form it was in at the time." -Bulbapedia
I dont think anything happens. Basically, skill swap is like the ability trace. Trace doesn't work on another trace or forecast, so I don't think it would work here. However, I don't know what would happen to the Castform.
