Changing Avg. Cost Calc. Type fro Item to Item,Location & Variant

Hi, We have been to Dynamics NAV 2016 from past 3 months. and most of our Items are on Avg. Costing. We noticed that Inventory Valuation showing negative Values for those Items having Zero Quantity. We have been closing Inventory Periods every month.So my Question is changing the Inventory Setup from Item to Item,Location & Variant for Avg. Cost calculation will affect prior months as well? Is it ok to change the value of this Field during middle of the Inventory Period.

Thanks, Sunny