Upgrade Cognos without upgrading Reports

[cognostechie]

There is a company where the BI manager always installs the latest version of Cognos as soon as it is released but never upgrades the reports. He didn't even know that reports should be upgraded until I told him. Most of the reports are written in version 8x and 10x. While IBM says that they would run in newer versions even if not upgraded but this time I saw some reports stopped running showing syntax errors in the latest version while the same reports are running in the prior version.

Any thoughts? It maybe that the SQL generated in the latest version is slightly different than the earlier one.