ds4/src/module/migrations/001.ts

export async function migrate(): Promise<void> {
    for (const a of game.actors.entities) {
        const updateData = getActorUpdateData();
        console.log(`Migrating actor ${a.name}`);
        await a.update(updateData, { enforceTypes: false });
    }
}

function getActorUpdateData(): Record<string, unknown> {
    const updateData = {
        data: {
            combatValues: [
                "hitPoints",
                "defense",
                "initiative",
                "movement",
                "meleeAttack",
                "rangedAttack",
                "spellcasting",
                "targetedSpellcasting",
            ].reduce((acc, curr) => {
                acc[curr] = { "-=base": null };
                return acc;
            }, {}),
        },
    };
    return updateData;
}