If blockers have already been declared when Stoneshock Giant's last ability resolves, those blocks won't change or become undone.
Stoneshock Giant
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Lars Grant-West
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
