I think 'we truly have free will' is already an assumption I'm not comfortable making, but even assuming that it is true, I don't think it necessitates that a person can change their work ethic just through will-power. I see work ethic as more comparable to a talent - if someone has an extremely poor work ethic it is extremely unlikely that they will ever jump to the 99th percentile in work ethic even if they really want to be conscientious, just like someone who is an awful musician is very unlikely to ever become an incredibly good musician just through willpower.