The Supreme Court of Canada ruled on Monday that divorced or separated parents must report increases in their income when it comes to paying child support. And those who don’t increase their child support to keep pace with their income could face significant retroactive orders to pay up.

However, the judges leave plenty of room for lower court decisions to vary based on the facts of each case.

The ruling stemmed from four Alberta fathers who challenged the retroactive awards imposed on them. The SCC ordered two of them to pay up, while the other two were absolved.

The legal principles involved extend far beyond the four cases and could affect thousands of couples.

Federal law sets guidelines that tie child support to ability to pay and number of children.

Nevertheless, people leave jobs and take new ones througout the years. Custody arrangements also change as children move back and forth between parents, so support payments often become out of date.

In practice, though, the onus has typically fallen on spouses receiving child support to go to court if they think they’re owed more money. But many say they couldn’t do so if their exes haven’t disclosed their new incomes.