While Fedup has improved a lot since its start, Fedora developers have determined "the current design is unsupportable" so they're planning a new approach. The new Fedup will likely download all packages for the new Fedora system followed by using systemd's "Offline Updates" feature to install all of the new packages. This would be done likely in conjunction with PackageKit to make a nice user experience.
Will Woods of Red Hat has already prototyped a new Fedora upgrader system using this new design as a DNF plug-in. More details on the Fedup changes planned for Fedora 23 can be found via this Fedora development list post.