You don't need to start from scratch to make changes to your will. You can modify your will by simply attaching new directions to it. You can also attach a list identifying your personal belongings and to whom you are leaving them. Each state has its owns laws regarding estate planning, and an attorney can answer specific questions about attachments to your will.
Generally, state laws allow you to distribute specific personal property by attaching a distribution list to your will. The list may identify the property and person you are leaving it to. For example, you may state in an attached list that your stamp collection goes to your brother, John. The list may be completed when you write your will, or you can do it later. State laws often require wills to refer to your right to attach personal property lists, if you want to use such a list.
A codicil is an attachment to your will that changes its terms in some way. Codicils may add or subtract entire will provisions or simply tweak existing terms. For example, you can state in a codicil that your fishing cabin goes to your friend, Charlie. Or you may change the guardian for your minor children from your sister to your mom. Codicils can accomplish such changes without the need to redraft your entire will. Generally, codicils also clarify that all original will provisions not mentioned in the codicil remain unchanged.
Generally, you must sign both codicils and distribution lists in order to make them valid and enforceable. You are typically not required to sign a distribution list for personal property in front of witnesses. However, codicils demand all the same formal requirements as original wills. This generally includes signing before two adult witnesses. Most states require that your witnesses be at least 18 years old and competent, and they cannot typically have an interest in your will.
It is important to date all codicils and distribution lists. Courts enforce your most recent, valid estate planning documents when you die. For example, suppose your will is dated July 20, 2013. A judge must be certain a codicil is intended to change this July 2013 will -- and not an earlier version of your will -- before he enforces it. He cannot make such a determination if your codicil is undated. Likewise, if you make changes to a distribution list, its date is critical to establish revocation of your prior wishes.