BEA attempts to support binary and source-level compatibility between WebLogic Server 8.1 and WebLogic Server 6.1 in the areas of persistent data, generated classes, and API compatibility. In some cases, it is impossible to avoid incompatibilities. Where incompatibilities arise, they are fully documented in the Upgrade Guide for BEA WebLogic Server 8.1.

Persistent Data Compatibility

WebLogic Server 8.1 does not require you to make any changes to WebLogic Server 6.1 persistent data storage, including configuration files, deployment descriptors, and JMS messages.

Generated Classes Compatibility

WebLogic Server 8.1 does not require you to recompile applications in order to create new generated classes.

API Compatibility

WebLogic Server 6.1 applications deployed on WebLogic Server 8.1 will function without modification. Exceptions to this rule include cases where API behavior was changed in order to conform to a specification or to fix incorrect behavior. In certain circumstances, this correction may cause your application to behave differently.

Protocol Compatibility

Interoperability between WebLogic Server 8.1 and WebLogic Server 6.1 is supported in the following scenarios:

A WebLogic Server 6.1 client can invoke RMI-based applications hosted on a WebLogic Server 8.1 server using IIOP, t3, t3s, http, and https. JMS applications can be invoked using t3, t3s, http, and https.
A WebLogic Server 8.1 client can invoke RMI-based applications hosted on a WebLogic Server 6.1 server using IIOP, t3, t3s, http, and https. JMS applications can be invoked using t3, t3s, http, and https.
A WebLogic Server 8.1 web server plug-in can proxy to a 6.1 server

Notes:

Mixing WebLogic Server versions within the same Administrative domain is not supported.
Servers within an Administrative domain can be at different Service Pack levels as long as the Administration Server is at the same Service Pack Level or higher than its Managed Servers.
All servers within a cluster must be at the same Service Pack level.