切替
きりかえ · kirikae · JLPT N1 · noun
switchover
切 — 七 (phonetic) + 刀 (knife radical)
切 is a semantic-phonetic compound: 刀 'knife' gives the meaning of cutting, while 七 supplies the sound. The original sense was to slice or cut with a blade. From there it extended figuratively to 'close to' (as in pressed against) and 'urgent/earnest', as in 切实.
替 — two 夫 (man) over 曰 (say)
Originally written with two 立 (stand) over 白, depicting two people side by side, suggesting replacement or alternation. The form evolved to two 夫 over 曰. The core meaning shifted from 'decline/abolish' to 'replace, substitute for'.