I would not suggest you change your existing will in this manner without an attorney. You may very well invalidate your entire will by doing so. While an attorney could add a codicil to your will, it would probably be a much better choice to revoke your current will and write an entirely new one in order to ensure your wishes are carried out.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I agree with Ellen. A mistake in the drafting or execution of a will or codicil may produce some veru undesirable results. Better to have a professional do it.