〜を機に
taking the opportunity of, on the occasion of
Noun + を機に / を機として
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を機に means using an event as the opportunity to make a deliberate change: 退職を機に、田舎に引っ越した, "when I retired, I took the chance to move to the countryside".
The event is usually a milestone in life or business: retirement, marriage, a new job, a hospital stay, an anniversary. The second half is a decision someone made because of it.
It's a formal relative of をきっかけに and を契機に (N2). をきっかけに can be accidental or even negative, while を機に sounds deliberate, the way you'd explain a choice.
Compare を境に, which just marks a before-and-after line without implying a decision.
Examples
退職を機に、田舎に引っ越した。
When I retired, I took the chance to move to the countryside.
入院を機に、たばこをやめた。
Being in hospital was my chance to give up smoking.
結婚を機に、家を買った。
We bought a house when we got married.
転職を機に、生活を見直した。
Changing jobs was my chance to rethink my lifestyle.
開業十周年を機に、店を改装した。
We renovated the shop for our tenth anniversary.