Old Vauqun/Sound Changes

Old Čeuň is a direct descendant of Proto Čeuň-Adzovŭd, and due to it's location at the easternmost extreme of PČA languages it earns the namesake of the proto language, along with its westernmost sister Proto Adzovŭd. Note that IPA will be used for all transcriptions of the languages in question, regardless of the phonemicity of their standard writing systems.

Čeuň-Kfimea Dialectal
PČA maintained a distinct Čeuň dialect for some time before the split. Various isoglosses are shared with P Kf (esp. E Kf languages). These dialectal rules include:

Expansion of I-Umlaut

[j] now triggers an I-Umlaut (explained in further detail on the PČA morphophonemics page)), only affects the syllable immediately preceding the [j], unless an [r] or [w] is in the same syllable as the target vowel in which case the umlaut does not occur: [a æ o u] > [æ e e ɨ] / _$(C)(V)j !_...[rw]...j

U-Umlaut
 * togju > tegju
 * fatjo > fætjo
 * but nawkjo > nawkjo

[u] now triggers a rounding umlaut in the same conditions [i] triggers I-umlaut (long or word-final): [a æ e i ɨ] > [ɔ œ ø y ʉ] / _...u: OR _...u#. U-umlaut affects both short and long vowels, and it is blocked by [j] & [r] in the same syllable as the target vowel [w] behaves the same as [j]
 * e:vku > ø:vku
 * kɨnũːt > kʉnũːt
 * aju > ɔju
 * but sjaθu > sjaθu
 * enweθk > ønweθk
 * but ajsiwte > ajsiwte

Rounding of ɨ

ɨ became ʉ when preceded by [x] [γ] or [w] and when followed by [v] [f] [m] or [w]

Palatalization of Velar Spirants
 * gnɨ:vo > gnʉ:vo
 * xɨ > xʉ

[x] [γ] (which might have had the allophones [ç] [ʝ] in palatal environments already) became [ʃ] and [ʒ] before [i i: e e:] and [j]. This became phonemic when various vowel shifts occurred later (after the era of PČA unity) Assibilation of Dental Fricatives
 * suxidvẽ > suʃidvẽ

Dental fricatives become their respective alveolar sibilants in root-initial consonant clusters. Both Čeuň and Kfimea languages feature further reduction of the dental fricatives but other than this the sound changes are largely handled differently by each language group: [θ ð] > [s z] / #_C
 * θke > ske
 * ðgaju > zgɔju