Assuming you have defined split valuation types in the sap img, you must remove all stock for the current period and one previous period before you can create the material with different valuation types.
The best method is to wait until the last day of a period, and then perform a physical inventory adjustment to remove the stock.
Then on the first day of the next period you can set up the material accounting views for each split valuation type.
After the material valuation types are set up for the material, you can do an inventory adjustment to put the stock back in.
Make sure you notify your accounting inventory control departments as to the quantities and values you are taking out at
the end of the period.