One reason to update a will may be that the guardian you initially chose is no longer appropriate, able, or willing. One obvious problem would be that a guardian is deceased now, where as he/she/they were not when the will was written. In this situation, you may have designated an alternate guardian, which is recommended. However this is not always the case. Sometimes there are situations that you cannot solve by writing into a will an alternate guardian.
For instance, perhaps your parents that you originally designated have become much older since the last time you wrote a will and so have your children. Maybe you doubt if elderly parents taking care of a willful teenager or multiple willful teenagers is in the best interest of your children or your parents. Additionally, maybe you believed your parents or guardian could have taken care of the children into their teen years, but now a serious health issue has arisen, and you no longer think it is possible for them to take care of the children. Perhaps now, your much younger sister or brother who was too young at the time of the last drafting of a will, has a family and would be the more appropriate placement.
Lastly, sometimes you simply have a falling out with the guardians and think someone else might be a better candidate. Relationships change throughout the years. This is why it is important to take a look at your will and consider updating it.
Tip: The recommended time period for updating wills ranges between 3 and 5 years. This is recommended not only by attorneys but also by financial advisors.