At this point it is only possible to define the custom buildback once upon creation. In order to switch the buildpack someone has to create a new app.

However everytime someone pushes his code changes the newest version of the buildpack repository is fetched. So a workaround is to update the actual buildpack repository - it is a good practice to fork the desired builpack and have the buildpack URL pointing to this one. It is possible that the buildpack URL points to a specific branch, tag or a specific commit.