If I understand this you want your website to rank for widgets but Google is ranking you for sprockets.
What specifically have you done to change the topic of your website?
What did you do with your old content?
What have you done for new content?
What did you do with your old backlinks & their anchor text?
What have you done to develop new backlinks & different anchor text?
Have you checked the competition levels of the new keyword? Maybe your site is only strong enough to rank for widgets and the sprockets industry is just more competitive.