Hi,How will the two options coexist? Will the author have option to select the best answer AND the community will get to vote the best answer? If a user has selected a best answer will he be able to change his choice, if yes then how?
In my opinion,
Letting the author choosing the "Best Answer" making no sense, but. another option would be to allow 2 options for the admin.
1. Allow the thread author to mark the "Best Answer" only.
2. Allow specific user-groups to mark the "Best Answer" only.
3. Allow to both thread author and selected user-groups.
And yes,
If some one selected "Best Answer" he can revert. just like Like / Unlike.