[Findegil]

Galin, may be we overanalyse the text her but the case is interresting:

In J.R.R. Tolkien last version of the text in the Quenta Noldorinwa we read, that Dior went to Doriath after the fading of Luthien and the death of Beren. We have to asume that he took the Nauglamir with him. Then we read:

But Dior wore the Silmaril upon his breast and the fame of that jewel went far and wide; ...
Yet the sons of Fëanor gained not the Silmaril; for faithful servants fled before them and took with them Elwing the daugther of Dior, and she escaped, and they bore with them the Nauglafring, ...

QI then reads:

The dwelling of Elwing at Sirion's mouth, whre still she possessed the Nauglafring and the glorious Silmaril, beame known to the sons of Fëanor; and they gathered together from their wandering hunting-paths. But the folk of Sirion would not yeild that jewel which Beren had won and Lúthien had worn, and for which faair Dior had benn slain. ... And yet the sons of Fëanor gained not the Silmaril; for Elwing cast the Nauglafring into the sea, whence it shall not return until the End; ...

QII has story much nearer to the one told in Sil77:

Upon the havens of Sirion new woe had fallen. The dwelling of Elweing there, where still she possessed the Nauglafring and the glorius Silmaril., became know unto the remaining sons of Fëanor, Maidors and Maglor and Damrod and Díriel; and they gathered together from their wandering hunting-paths, and messages of friendship and yet stern demand they sent unto Sirion. But Elwing and the folk of Sirion would not yield that jewel which Beren had won and Lúthien had worn, and for which Dior the Fair was slain; and least of all while Eärendil their lord was in the sea, for them seemed that in that jewel lay the gift of bliss and healing that had come upon their houses and their ships.
... And yet Maidors gained not the Silmaril, for Elwing seeing that all was lost and her child Elrond taken captive, eluded the host of Maidors, and with the Nauglafring upon her breats she cast herself into the sea, and perished as folk thought.
But Ulmo bore hre up and he gave unto her the likeness of a great withe bird, and upon her breast there shone as a star the shining Silmaril, as she flew over the water to seek Eärendel her beloved. ...
... He stood now most often at the prow, and the Silmaril he bound upon his forehaed; ...

The only later source we have beside Bilbo's Song is the Tale of the Year. In that the percious thing is named Nauglamir only when the necklace is made and when the Ruin of Doriath by the Dwarves is told. There after is only the Silmaril mentioned.

In Sil77 the last mention of the Naulglamir is when Dior wears it:

The Dior arose, and about his neck he clasped the Nauglamir; ...

But now the rumour ran among the scatterd Elves of Beleriand that Dior Thingol's heir wore the Nauglamir, and they said: 'A Silmaril of Fëanor burns in the woods of Doriath'; and the oath of the sons of Fëanor was waked again from sleep. For while Lúthien wore the Necklace of the Dwarves no Elf would dare to assial her; ...
...
... But the sons of Fëanor gained not what they sought; for a remnant of the people fled before them, and with them was Elwing Dior's daughter, and they escaped, and bearing with them the Silmaril they came in time to the mouths of the River Sirion by the Sea.

Thereafter only teh Silmaril is mentioned even so "wearing the Silmaril on her breast" does suggest that the stone is still bound in the Necklace.

So it is clear that Christopher Tolkien changed the use of Nauglamir and Silmaril in naming the treasure deliberatly form what he found in the source texts. But the only hint we have for the reason might be JRR Tolkiens use in the Tale of the Year.

Respectfuly
Findegil