Once the policy is in effect, you need to submit a new application and go through the whole underwriting process again for upgrade/ downgrade of insurance plan.
Tips from Bowtie: you are highly recommended to keep your existing policy before the new policy is in effect. Once your new policy is effective, cancel the other one! In this way, a window period is avoided and your protection is secured.
Window period means the period when you are not protected by insurance as your existing policy is cancelled but new policy is not yet in effect.