upgrading Concrete 5
Version 5.7 is a complete rewrite and 8.x continues from that point. So the jump from 5.7.x to 8.x is quite minor. 5.6 to 5.7 (or 8.x) is a whole different story. So you'd have to migrate and make sure the Add-Ons you use will have a 5.7 version (or 8.x version).
I'm guessing a rewrite of the site has to be done or quite some work has to be put in to transform it into the latest version!