Hi Lucas ;
For me, the best way to describe reasons of effects , OMSK tcode must be analyzed.
Please try to solve relations about Account Catagory Referance and Valuation Class and Material Types.
You asked a question , my answer :
1- If you changed material type of material , you can or not have some valuation class. Please look at OBYC tcode. But also, you can have same valuation class related with combination rule.
Valuation class :
*The valuation class is used to determine which stock account is updated during the goods movement of a material.
*The valuation class enables you to manage the stocks of multiple materials in a stock account.
Regards.
M.Ozgur Unal