Explanation
According to the Salesforce Manufacturing Cloud documentation1, if you modify the forecast display settings, such as the forecast start period, the forecast frequency, or the forecast display duration, all active forecast records expire, and the regenerated records replace them. These actions trigger the regeneration of all the eligible accounts that satisfy the forecast generation criteria. Updating the forecast adjustment period or the forecast formula does not affect the forecast display settings, and therefore does not trigger the regeneration of account forecasts. References: Configure Forecast Generation and Display Settings