Nothing happens to the accounting, only the reporting for the classes involved.  The new class used would increase its reporting value, and the old class reporting value would lower

Resolution for 'classes involved issue: 'What happens if I change the site location on a bill from dropship to another location?' available: Yes.

The classes involved 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 classes involved issue yourself by looking for a resolution described in the article. If it is a complex classes involved issue or you are unable to solve the issue, you may contact us by clicking here or by using other classes involved support options.

For more information about resolving issues related to classes involved in Quickbooks/QBO or to get support for the issue 'What happens if I change the site location on a bill from dropship to another location?', click the link below: