Hi Satya,
TestComplete is backward compatible in the terms that it can open test projects from the old versions and convert them. Once the project is converted it cannot be used in the old version.
However, conversion touches mostly configuration files (like project suite, project) and, maybe some other files like NameMapping. Script units, files from the Stores, pictures remain unchanged. So, depending on the contents of your project, if you really need to use different versions of TC with the same project, you can create project suite and projects in the specific version of TC and then add project items as existing ones (via Add | Existing Item... menu command).
