It is impossible to change the mission of an online community because the mission is an immutable characteristic of the community.
Community is a way to solve a specific social problem. The mission describes the solution you propose. People become part of your community because they care about the problem being solved and believe in the solution you are proposing. The mission is what brought them to your platform and united them in a community.
Please note that it is impossible to change the mission of a community without destroying the community. If you see the need for a new mission, you need to create a brand new separate community.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.