There are photos of the original tower and the current tower, both with and without the lantern room here:

http://www.uscg.mil/hq/g-cp/history/WEBLIGHTHOUSES/LHHI.html

There is a write-up on the lighthouse here:

http://www.hawaiiweb.com/html/barbers_point_lighthouse.html

According to this write-up the light was automated in 1964. The NPS site states that the present optic was also installed that year. I would guess that 1964 was the year that the lantern room was removed.