Per far above, I have actually tried:
update_sub_field(field_64c782fc4c55d, 84, 1074);
And it does not update even if the value is different from the current value.
According to the man page, $selector can be a string, too,
$selector (string|array) (Required) The sub field name or key, or an array of ancestors and row numbers.
$value (mixed) (Required) The new value.
$post_id (mixed) (Optional) The post ID where the value is saved. Defaults to the current post.
