An Application/Product developed using one version of a platform should still work in a newer version of a platform. The Testing that ensures new version of the product to continue to work with the older product is known as Backward compatibility testing.
A user has created a very complex excel sheet to track project schedule, resources, expenses using Excel 2000. The user then upgrades his MS Office to 2010 version. The functions that were working on MS Office 2000 should still work which means assets created using older version should continue to work.
In case if the assets created using older versions do not support new versions due to any reason, then proper migration path should be given to the user so that they can migrate smoothly from prior version to current version.