Answer
No, on two separate counts — a field that's part of the entity's key (like Lot/Batch Number on Inventory Part In Stock) can't be modified this way since key fields are immutable by design, and separately, a Custom Event's "Before" trigger generally can't be used to update the data it's triggered on at all, for underlying technical reasons tied to how before-triggers execute relative to the actual data write; if similar cleanup/transformation logic is needed, it has to happen before the data reaches this point (in the source system or an earlier processing step) rather than inside a before-change custom event.