Is there a way to update a Lead Owner once they have been converted?

Is there a way to update a Lead Owner once they have been converted?

I have mass imported leads to our CRM (on behalf of my organisation) and by default was selected as the Lead Owner. I then discovered these leads were existing accounts, so I have converted them accordingly. 

Once I converted the leads I changed the contact owner to the correct person owning the account, however when I set up dashboards on my home screen I can see it looks like I have converted 80% of the organisations leads.


Is there a way I can change the lead owner so our dashboard statistics are accurate? I don't want to have to start again as I've just spent 2 days cleaning up and validating all the data I've imported.