QB Issue Resolution:

Once you open the file on the new version, you cannot open it anymore on your old version. The only way to access it again on the old version is to use the back up that was created before you updated or opened it on the new version. If you will upgrade to the new version, you need to update all company file/s to it.

Resolution for Issue 'Is it possible to have v12 users and v15 users using same company file?' available: Yes (Solved).
Source: Intuit Community forum.

Is it possible to have v12 users and v15 users using same company file?: 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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