Unfortunately, there is no migration path. The new Websitebuilder is too different from Sitebuilder to allow this, so you do have to create a totally new site. Fortunately, in your case the site is not too large and should not take too long. You should even be able to copy/paste text off the old published site and into the new builder. You will need to do some formatting, etc to fit your eye, but it should be pretty easy.