From what little I can find, it appears that it was Boffer who initially wrote on this board that the Oriental circlet was altered in 1858 because some of the diamonds belonged to the Hanoverian claim and, at that time, it was decided to shorten the tiara. Unfortunately, Boffer did not provide any supporting evidence for his assertion. See http://members2.boardhost.com/royal-jewels/msg/archive/1340930741.html
I am happy to work with the evidence we do know for sure.