This knowledge base article explains why within the Inventory preferences the weight and volume UOM fields are locked down as shown below.
The Weight UOM is locked down once an inventory item has recorded a weight amount greater than zero and also has the weight UOM specified. This is also the same for the Volume UOM, this too is locked down in the inventory preferences once an inventory item has recorded a volume amount greater than zero and has a volumne UOM specified.
This inventory item above will lock the inventory preferences for Weight and Volume UOM.