31 CFR §363.143

You may purchase an eligible security by redeeming all or a portion of your zero-percent certificate of indebtedness and applying the proceeds toward the purchase of another eligible security. To do this, your zero-percent certificate of indebtedness must be of sufficient value to cover the cost of the security. If you are paying for a security using the redemption proceeds of a zero-percent certificate of indebtedness, you must pay the full amount of the purchase price of the security using the redemption proceeds.