When a WTPart is added to a bom structure, the unit in usage link (Part usage attribute) is set same as default unit of WTPart. But if the WTPart is replaced, the unit continues to be the older one, instead of replacing unit as per the new WTPart.
However, for a single part replacement, the issue is addressed by using a preference setting. But if the user choose to replace part using mass replace functionality, there is no alternative in the system to replace the unit in bulk when a Part is replaced in multiple bom structures.
The attachment has the detailed steps of issue and mass replace functionality.