WebSphere® Application Server Version 7.0 is interoperable with other versions of the product under certain conditions.
Read Overview of migration, coexistence, and interoperability and Premigration considerations. For resources to help you plan and perform your migration, visit Knowledge Collection: Migration planning for WebSphere Application Server.
WebSphere Application Server Version 7.0 is generally interoperable with Version 5.1.x and Version 6.x. However, there are specific requirements to address for each version. In general, you should be at the most recent group PTF level to support interoperability.
You can upgrade a portion of the nodes in a cell to WebSphere Application Server Version 7.0 while leaving others at the older release level. This means that, for a period of time, you might be administering servers that are at the current release and servers that are running the newer release in the same cell.
This information is dynamic and might be augmented by information in technical articles that are available on the IBM® DeveloperWorks WebSphere site. Check the site for the latest information.