Domain name has become irrelevant

Can I change it without losing search rankings?

[Automotive site]

I feel the site has gone in a direction that has made the domain name irrelevant and off-topic. I want to change this domain name either into something related to the content of the site or something brandable.

How do I change the domain name without losing rankings? This site gets couple hundred daily uniques and is around 15 months old. I don't want to start all over again. The site is in Wordpress. I feel this will be a good move for the site moving forward.

What will happen to the urls of old content? Will those rename to follow the new domain i.e. newwidget.com/category/article title

I am assuming the urls of new content will follow the new domain.

[Sgt_Kickaxe]

You'll want to leave the old urls in place but have them forward to new urls on your new domain using 301. You should know that you might have to leave the old domain up with redirects for a very long time, especially if you had incoming links pointing to it. Also, expect some rankings loss since 301 redirects do not carry 100% rank value. Having done this before I suggest adding the new urls to your sitemap but also leave the old urls in place and do not "noindex" anything, let search sort it out. You need crawlers to see everything, if they don't find your 301's because you block them you won't get credit for the move. When Google returns zero results for your old domain name it's safe to remove the old urls from your sitemap.