1) Change 2) Diphthongise 3) Shift 4) Switch 5) To become a diphthong 6) To change to a diphthong 1) Diphthongise 
• (v. t. & i.) To change into a diphthong, as by affixing another vowel to a simple vowel.
Diph'thong·ize transitive verb & i. To change into a diphthong, as by affixing another vowel to a simple vowel. 'The diphthongized long vowels.' H. Sweet. 
[v] - change from a simple vowel to a diphthong
diphthongise verb change from a simple vowel to a diphthong; `This vowel diphthongized in Germanic`