I don't use plymouth.
Can fedup be used to go F17 -> F18
without having it used\installed.
Asking as it's was the last entry in a bz I had:
https://bugzilla.redhat.com/show_bug.cgi?id=880679
From the bz, was\is there anything I may have overlooked.
(also tested with --disablerepo=* --enablerepo=fedora)
Frank