If you are generating statistical forecasts using Demand forecasting (with Azure Machine Learning) within the D365 UI, and not the new Demand planning app, you would have to import data for the new item using the data entity: " Historical external demand for V2 entity" to simulate previous demand. You would have to do this every period until D365 has enough historical demand data to use in the forecast generation.
The Demand forecasting tool does not have specific functionality for this, but it does allow manual adjustments to the forecast in Excel, which it suggests can be used in these scenarios. The old PDF from the AX days describes (although I have not tried if it looks the same in F&O). You can find the PDF linked at the bottom of this page.
How are you doing the thing you call 'run forecast'?
If you are using the Demand Planning tool, it has the capability for 'phase in/phase out', but I do not believe this uses the Alternative Item field.
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