In both Sindarin and Noldorin the primitive diphthong [oi] became [ui]. Tolkien described this change in a chart of the phonetic development of Primitive Elvish diphthongs in all three languages (Quenya, Telerin and Sindarin), appearing in notes associated with Tolkien’s discussion of Hands, Fingers and Numerals from the late 1960s (VT48/7). It also appeared as an (Old) Noldorin development in the Comparative Tables from the 1930s (PE19/25). There are a fair number of examples in both Sindarin and Noldorin:

• ✶ō-yā [> *-oia] > S. -ui “-ful” (VT42/25).
• ✶oio > S. ui “ever” (Let/278).
• ᴹ√GROJ > N. gruin “ruddy” (Ety/ROY²).
• ᴹ✶poikā > N. puig “clean” (Ety/POY).
• ᴹ√MOY > ON. muina > N. muin “dear” (Ety/MOY).

As shown by the last example, this was an early sound change, in the Old Noldorin/Old Sindarin period. In fact, where the diphthong [oi] arose later from the vocalization of spirants, it became [oe] in Sindarin (not so in Noldorin, where the vocalizations produced [ou] which then became [au]). As discussed elsewhere, Tolkien frequently represented the [ǭ] that developed from [ā] as ō in Noldorin, and at this conceptual stage the diphthong ōi (or [ǭi]) also developed into [ui]:

• ᴹ✶wāyā > wōia > ON. uia > N. ui “envelope” (Ety/WAY).
• ᴹ✶nāyǝ > noi > ON. nui “lament” (Ety/NAY).
• ᴹ✶istāyā [> *istǭio] > N. istui “learned” (Ety/IS).
• ᴹ✶m(b)auy- [> *mbǭi-] > N. bui “I must” (Ety/MBAW).

This last example shows that in Noldorin, [ǭi] from various origins developed into [ui], since during the 1930s [au] also became [ǭ] in Old Noldorin. There are examples indicating that the Sindarin phonetic developments of [ǭi] were not the same as Noldorin, however, most notably:

• ✶gāyā [> *gǭja] > S. goe “terror” (PM/363).
• ✶wāyā [> *wǭja] > gwoe > S. gwae “wind” (PE17/34).

These two examples indicate that [ǭi] ultimately became [oe] in Sindarin, just like later diphthongs [oi] produced from spirant vocalizations. The most straightforward explanation is that after the primitive diphthong [oi] became [ui] in Sindarin, the diphthong [ǭi] (or [ǫi]) became [oi] and eventually merged with the phonetic development of later diphthong [oi]. David Salo noted these differing phonetic rules for Sindarin and Noldorin in Gateway to Sindarin (GS/§4.190, §4.229).

There is one pair of examples that hint that Tolkien considered having primitive [oi] become [oe] more generally in Sindarin:

• √SLOY > lhoew “poison” and lhoer “venom” vs. Q. hloima and Q. hloirë (PE17/185).

Absent further evidence, it’s hard to guess what Tolkien intended here.

Conceptual Development: In the early chart of Gnome Vowels from the 1910s, the diphthongs oi produced ui when stressed, or we/i when unstressed (PE15/13). As pointed out by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§1.2), the clear examples of primitive [oi] all became [ui] in the Gnomish Lexicon:

• ᴱ√KOẎO > G. cuith “life” (GL/27) vs. ᴱQ. koire (QL/48).
• [*βoine >] G. buin “desire, wish” (GL/24) vs. ᴱQ. voine (QL/60).
• ᴱ✶ı̯ōı̯ē > G. gui, past tense of gôtha- “to have” (GL/42).

The same is mostly true of the Early Noldorin of the 1920s:

• ᴱ✶tloise > ᴱN. tlui “slender” vs. ᴱQ. toloise (PE13/154).
• ᴱ✶koile > ᴱN. cuil “life” vs. ᴱQ. koile (PE13/141).
• ᴱ√POYO > G., ᴱN. puig “clean” vs. ᴱQ. poika (GL/64, PE13/124, QL/75); compare N. puig above.

As noted by Roman Rausch in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923 (HPITN/§4.2.4), there are a few examples of [oi] > [ai] in Early Noldorin. All of them seem to be derived from the combination [oiw]:

• ᴱ✶tloiwe > ᴱN. tlaiw “sling” vs. ᴱQ. toloime (PE13/154).
• [*koiwa >] ᴱN. caiw “lively” vs. ᴱQ. koiva (PE13/140).
• [*oiwe >] ᴱN. aiw “bird” vs. ᴱQ. oive (PE13/136, 158).

Despite the minor variations noted above, it seems that Tolkien generally had primitive [oi] > [ui] throughout all conceptual stages: Gnomish, Noldorin and Sindarin.

HPG/§1.2