March of Dimes was created to deal with polio, and was left without a mission once a vaccine was developed. So with a nationwide infrastructure in place, it decided to change its mission from polio to premature children. Today, if a 501(c)(3) wants to broaden or change its mission it doesn’t need to file a new 1023 and seek a new determination letter. Instead, all it needs to do was notify the IRS on its annual 990. On the state level, it will need to amend its Articles of Formation to reflect the new purposes of the organization.
Use of donated funds is a bit more complicated. If funds already raised are restricted, it will not be able to use those funds for the new charitable purpose. It would be better to raise new donations for the new purpose and segregate them from existing funds.
The bottom line is a charity can change and evolve. It doesn’t have to start over.