Group Policy Preferences are great. It's nice to pre-set things for your users and know they can change it if they have a need.
This is a PITA to test though, as you can't re-apply the preference to your test machine.
Here's the fix.
Open up the regkeys
The GUIDs of the applied GPOs will be recorded here. Delete them and reboot. Viola, your GPPs will reapply like magic.