Changing Item Cost While WO Being Completed

Hi all

I have searched but couldn't find anything definitive although I think I know what the answer is...

We had a user who raised an issue that the Actual Variance of a WO was incorrect based on the requested and issued quantity of a component on the parts list, as the requested and completed quantity of the parent item were the same.  I found that there were issues and completions before and after a cost change, meaning that some transactions were at different costs to others, leading to the 'incorrect' variance calculation.  This issue has raised the question around the suggested process to update item costs while WO's are open or being issued to or completed against to