Changes in Inventory Valuation Method

CFAI text Vol 3 P.419 5.1 Changes in Inventory Valuation Method

At the end of first paragraph, it is stated :

An exemption to the retrospective restatement requirement applies when it is impracticable to determine either period-specific effects or the cumulative effect of the change.

I don’t understand what above statement really means. Anyone can explain clearly ? Thanks !