I did a network upgrade from Fedora 18 to 20 the other day and it went fine.
I ran this to get the Fedora 20 GPG key:

su -c 'yum --enablerepo=updates-testing update --advisory=FEDORA-2013-23598'

(from https://fedoraproject.org/wiki/Common_F20_bugs#Upgrade_from_Fedora_18_with_fedup_0.8_fails_due_to_GPG_key_problems)

Then followed the instructions: