Roll Up Standard Cost when Item already has manually entered Standard Cost

Summary

Roll Up Standard Cost when Item already has manually entered Standard Cost

Content

Item A is manufactured item with a work definition. I created Cost Scenario 1 and manually added a Standard Cost for Item A e.g. $3000. When I Roll Up Costs for this scenario, the system uses the manually entered cost instead of rolling up the cost. This is good. This is what I want.

However, 6 months later the business wants to remove this manually entered standard cost and roll up the cost for Item A from the work definition. How do I do this? I've tried creating a new scenario (Scenario 2) and doing a Roll Up, but it doesn't do anything because of the manually entered cost. I've tried deleting or editing the manually entered standard cost, but it won't let me delete because it's Published.