Should I edit a cost rule or add a new one?

Add a new version when the cost really changed, and edit only when the old value was wrong. A new version with its own start date ends the old one the day before. So your past keeps the old value, and only the days from the new date change.

Editing rewrites the past. That is right for a typo and wrong for a price rise. Say your supplier raises a price and you type it as an edit. Then every order you ever shipped looks more expensive, and last quarter's profit changes in front of you.

To add a version, open the rule, set Effective from to the day the new price starts, and save. For a product, type the new unit cost with its start date. On the Logistics page, do not use Add rule for a price change. Add rule makes a second rule, and both would charge your orders.

Deleting a rule is safer than it sounds, because nouz repairs the gap it leaves. And a cost that simply stops, like a warehouse you leave, should get an end date instead. Every order before that date then keeps its cost.