Yes, this is old, but here's an answer for anyone finding this thread who is in the situation I was in...

I got this error stating it could not upgrade from version 8 to version 6. But I know for a fact I had edited the package in the SQL2012 Version of SQL Data Tools, I uploaded it to a SQL2012 SSIS instance, it ran successfully from a SQL2012 test SSIS Server I uploaded it to a few weeks ago, and when I open the DTSX file in notepad is clearly said

<DTS:Property

DTS:Name="PackageFormatVersion">6</DTS:Property>

THE PROBLEM:

I installed SSMS 2014 last week, which I used to upload the dtsx file to SSISDB. Apparently, SSMS upgrades lower versions of dtsx files, or at least changes the PackageFormatVersion attribute.

THE SOLUTION:

Upload the dtsx file using the version of SSMS that matches the SSIS instance. When installing newer versions of SSMS, keep the old one on your workstation for tasks where SSMS is not backward compatible.