Sorry, Intuit does not offer upgrades, free or otherwise.  No QB program/file is backward compatible.  In order to share the file with QB 2016 on your main computer all other computers will need to be running 2016 also.

Also, you may have lucked out buying from eBay, but that is often […]

Resolution for '2015 computers issue: 'My company has 4 QB Pro 2015 licenses. I installed QB Pro 2016 on sales mgr computer. Need our 2015 computers to be able to use 2016 company files–cheap upgrade?' available: Yes.

The 2015 computers 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 2015 computers issue yourself by looking for a resolution described in the article. If it is a complex 2015 computers issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2015 computers support options.

For more information about resolving issues related to 2015 computers in Quickbooks/QBO or to get support for the issue 'My company has 4 QB Pro 2015 licenses. I installed QB Pro 2016 on sales mgr computer. Need our 2015 computers to be able to use 2016 company files–cheap upgrade?', click the link below: