RE: Changing item variants in inventory
Your best bet would be to use the Item Journal to do a negative adjustment and a positive adjustment (be sure to use the appropriate unit amounts / costs, posting date, etc.). I also suggest creating and using an appropriate Reason Code (may be hidden on the Item Journal by default, but you can toggle visibility from Choose Columns).
If it were any other type of reclassification, I would have recommended the Item Reclass. Journal (location, batch, dimensions, etc.). Unfortunately, the Reclass. Journal does not work for Variants; it has no "New Variant Code" field.
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