In Quenya, a velar nasal ñ [ŋ] appearing after n, ñ, l, r strengthened to g. Tolkien mentioned this change in the Outline of Phonology (OP2) from the 1950s:

Contact with ñ could thus only occur exceptionally in compounds. In these the initial m/mb, n/nd of the second element usually became m, n; and similarly ñ, ñg > ñ. This did not often occur, but in the known cases ñ remained in spelling (owing to absence from the alphabet of a letter for separate stop g) but the pronunciation became g actually after n (ñ), l, r. The best known case is Morñoþo > Morñot/s (pronounced Morgot/s, Morgos) contracted from Moriñgothŏ. (Cf. also such names as Argol(do) = “Noble Ñoldo”.) morĭ “dark” + ñgothō, “foe” (OP2: PE19/82).

This note was written in red and was thus a later addition to OP2, and the final etymology of this Quenya name for Morgoth was edited in green ball point (PE19/82 note #65), indicating these final edits date to around 1970. There was no mention of these sound changes in the Outline of Phonetic Development from the 1940s (OP1: PE19/47-48), so it seems they were a comparatively late idea. The only two example of this sound change are the ones given above.

As indicated by Christopher Gilson (PE19/82 note #65), the fact that one of these names is the Quenya equivalent of Morgoth is probably notable. In the Silmarillion Tolkien wrote:

Then Fëanor rose, and lifting up his hand before Manwë he cursed Melkor, naming him Morgoth, the Black Foe of the World; and by that name only was he known to the Eldar ever after (S/79).

It was probably important to Tolkien that Fëanor gave Melkor this name. This naming is problematic, however, because Morgoth is a Sindarin word and at the time of the naming the Noldor had not yet encountered the Sindar. In the 1930s and 40s, this would not have been a problem, since the “Sindarin” of that conceptual period was actually the native language of the Noldor, but this was no longer the case in the linguistic scenario of the 1950s and 60s.

Tolkien explained this in a footnote to his Silmarillion drafts in the 1950s:

In the ancient form used by Feanor it was Moriñgotho (MR/294).

This aligns with the etymology of Morgoth’s name given in the revisions to OP2 shown above. It is conceivable that Tolkien introduced these sound changes to make the Quenya pronunciation of the name Morñoþo [morgoθo] closer to that of Sindarin Morgoth [morgoθ].

@@@ maybe not in MQ.