I do know that the Old EQ UI was still available when Luclin came out. You could either use the Old "Classic" UI or do a full screen "Velious" UI which was basically transparent windows that you had the ability to change the color of and that was about it.
According to Allakahazam's
Patch History Page Luclin went live on December 4th 2001, the New Interface went live on July 24th 2002 and the Classic and Velious Interfaces were extinct on January 9th 2003
Which is about how I remember it happening because I distinctly remember the new UI not coming out until sometime after Luclin was released but before PoP was released.