How to remove Catalyst and use open source drivers?

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http://forums.fedoraforum.org – I'm trying to remove the Catalyst drivers and replace them with the open source driver xorg-x11-drv-ati. The open source driver is already installed, but when I remove Catalyst, rebooting hangs at the Plymouth splash screen. If I press Esc while Plymouth is still loading, I can see that the system fails at plymouth-quit-wait.service, and I can log in using a terminal. Reinstalling Catalyst fixes the Plymouth-quit-wait problem. So I think there's some configuration I need to change, but I have no idea what it might be, and all my searches seem to turn up is info on how to install Catalyst. Not (HowTos)