Tā yuánlái shì gōngchéngshī, hòulái gǎiháng zuò le chúshī.
He was originally an engineer and later changed careers to become a chef.
Tips
usage
改行 specifically means changing one's 行业 (trade or profession), not just changing jobs within the same field. Compare 跳槽 (to job-hop within the same industry).