If the sole reason for reducing work is lowering maintenance, it will not work. Since 2018, the Federal Supreme Court calls this malicious and abusive under Art. 2(2) ZGB. It excludes amendment regardless of whether hours were reduced or resignation arranged. If done solely to harm the former partner, maintenance is not reduced and hypothetical income is imputed: income that could reasonably be earned, generally the previous income.
If reduced hours are not intended to harm, consequences differ. Illness or operational need is not abusive and may permit an amendment claim. The court considers new income but also whether earning more elsewhere is possible and reasonable. If so, hypothetical income may still be imputed despite no malicious intent, immediately or after a period allowing higher earnings.
In short: If working hours are reduced solely to harm the former partner, lower income cannot justify reduced maintenance. If for other reasons, such as illness or operational need, the former partner may ask the court to amend maintenance. If earning more is neither possible nor reasonable, the court may grant the claim.