I was wondering the same thing and unfortunately having to rebuild the entire site is the way to go. Same for blog posts as they don't have a migration option which seems a bit ridiculous.
I've emailed Squarespace to find out how this will affect Google searches and SEO ... awaiting a response. This will not be good if it ruins all of that previous work. I work in Wordpress for another project and when they launched a new version it was seamless and didn't have to rebuild anything. Seems this wasn't done in the right way and they may lose a lot of customers or hold the rest back from the better options available in the new version.