Just as they did in Primitive Elvish, any y, w that came into contact with a following consonant became vocalic i, u and formed a diphthong with the preceding vowel. These Quenya-specific sound changes generally occurred after the Quenya Syncope. Examples include:

• ✶ayanā > aina “to hallow, bless” (PE19/79).
• ✶gayakā > aika “fell, terrible, dire” (PM/363).
• ✶glawarē > glaurē > ʒlaurē > laurē “sheen of gold” (PE19/79).

As indicated by the last example, this sound change must have been quite ancient, before voiced stops became spirants, but in all likelihood this remained an active phonetic rule for all of Quenya’s history. This sound change likely also applied to final y, w, but these were mostly vocalized already in Primitive Elvish.

Conceptual Development: In Early Quenya, these post-primitive sound changes were more common, since the phonetic inventory of Primitive Elvish in the 1910s included a number of weak palatal and labialized spirants [ç, ʝ, ɣʷ] that developed into y, w and then vocalized. In Qenya Phonology Tolkien denoted y, w that were derived from such spirants as ẏ, ẇ (PE12/15-16). He described their developments explicitly in many cases (with probable IPA values of the primitive sounds inserted):

χ̑ or þ͡y [ç] ... voiced to y̑ undergoing between vowels same changes as y̑ [ʝ] ... before voiced spirants or before semivowels it gave ẏ > ı̯ or i (PE12/18-19).

ẇ [ɣʷ] [became] w ... ẇ + consonant > u or ŭ [?u̯] (PE12/23).

ʒ̑ [ʝ] [became] y ... ẏ + consonant > i or ı̯ (PE12/24).

There are plenty of examples of this changes in the Qenya Lexicon; most roots containing Ẏ or Ẇ have at least one example.