QB Issue Resolution:

You can upgrade directly from QBES 12 to QBES 15 — Simply open the file in the new version of the software.

Resolution for Issue 'Can you upgrade Quickbooks Enterprise V12 to V15?' available: Yes (Solved).
Source: Intuit Community forum.

Can you upgrade Quickbooks Enterprise V12 to V15?: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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