Re: the name of this kind of transformation, in English, this might be called "monophthongization", or more generally as "vowel shift". It's not uncommon in human languages in general. It happened in ancient Japanese, producing the /e/ sound in modern 目{め} as a probable result of fusion of ancient /ma/ + nominalization particle /i/: /mai/ → /me/. It happened in Korean, such as where the modern vowel sound [에]{/e̞/} apparently evolved from a fusion of [어]{/ʌ̹/} and [이]{/i/}. It's happened in various places in the _evolution of Indo-European languages, such as modern Dutch "[heel]{/ɦeːl/}" from earlier Proto-Germanic "[hailaz]{/ˈxɑi̯.lɑz/}", or modern southern American English "[I]{/aː/}" from the "[I]{/aɪ/}" of most other varieties.

Re: the name of this kind of transformation, in English, this might be called "monophthongization", or more generally as "vowel shift". It's not uncommon in human languages in general. It happened in ancient Japanese, producing the /e/ sound in modern 目{め} as a probable result of fusion of ancient /ma/ + nominalization particle /i/: /mai/ → /me/. It happened in Korean, such as where the modern vowel sound [에]{/e̞/} apparently evolved from a fusion of [어]{/ʌ̹/} and [이]{/i/}. It's happened in various places in the evolution of Indo-European languages, such as modern Dutch "[heel]{/ɦeːl/}" from earlier Proto-Germanic "[hailaz]{/ˈxɑi̯.lɑz/}", or modern southern American English "[I]{/aː/}" from the "[I]{/aɪ/}" of most other varieties.