And here's a handy upgrade guide: http://www.projectatomic.io/blog/2017/11/fedora-atomic-26-to...
$ sudo ostree remote add --set=gpgkeypath=/etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-27-primary fedora-atomic-27 https://kojipkgs.fedoraproject.org/atomic/27
$ sudo rpm-ostree rebase fedora-atomic-27:fedora/27/x86_64/atomic-host
Static deltas are enabled from the last F26 commit to the F27 release which should reduce the amount of data you have to ship over. Just make sure your host is at the latest F26 release via `rpm-ostree upgrade` before doing the rebase operation.