how to store historical forecast data?

Summary:

Hi Guru,

we are currently running our supply plan drove by external forecast, our forecasts are uploaded using FBDI and every time when customer changes there forecast we will update our forecast accordingly by overwriting the original forecast quantity.

now there is an requirement from our sales department that they would like to analyze the customer forecast history to review the forecast fluctuation over time, but from what i know every time when we update our forecast(e.g. change the item A the forecast qty from 100 to 150 at April 1st), the historical forecast quantity will be removed from our supply plan.