The bulk is in EverQuest II\UI\DrumsUI
. Just delete the DrumsUI directory. The other bit would be in the EverQuest II\eq2.ini
Change the cl_ui_skinname to be the new Custom UI directory. No other files should have been over-written or modified.
Now, when you load up the character the first time it might look a little weird since the (server)_(chararacter)_eq2_uisettings.xml
file will have settings based on the previous UI. Window positions might be a little off and so on.