千变万化 (qiān biàn wàn huà) is a fixed phrase meaning 'constantly changing, ever-changing' — it focuses on the sheer number and frequency of changes, not on whether they are unpredictable. That's its key difference from 变幻莫测 (biàn huàn mò cè), which stresses erratic, impossible-to-foresee changes. 日新月异 and 千姿百态 are not direct synonyms: the former is about rapid progress, the latter about variety in form or style.