Valuation type cannot be changed (valuated stock already exists)

What causes this issue?

You have attempted to change the valuation type of the material.
Different valuation types normally mean that different prices can be
posted to different G/L accounts.
Since valuated stock of the material already exists, it is not possible
to change the valuation type in this case.

How to fix this error?

First post a goods issue for the existing valuated stock, then change
the valuation type, and finally post a goods receipt for the stock.

Error message extract from SAP system. Copyright SAP SE.