You can delete the line with the not in stock item, and then add the in stock substitute item.  While it sometimes works to just change the item line, sometimes it does not, right click on the line and delete or use menu edit>delete line and add what you need

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Resolution for 'Changing a Bill issue: 'Changing a Bill of Material Item for an existing Inventory Assembly' available: Yes.

The Changing a Bill issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Changing a Bill issue yourself by looking for a resolution described in the article. If it is a complex Changing a Bill issue or you are unable to solve the issue, you may contact us by clicking here or by using other Changing a Bill support options.

For more information about resolving issues related to Changing a Bill in Quickbooks/QBO or to get support for the issue 'Changing a Bill of Material Item for an existing Inventory Assembly', click the link below: