An ES 4.6 (the only thing that is 5.0 is the old broker) application can publish to or subscribe from a 6.0.1 or a 6.1 broker.
Lets say you have an integration where system A is sending data to system B. So you have ES/EI app the gets data from A, publishes a document, which is subscribed to by another ES/EI app that writes data to B.
You can upgrade your wm environment, which will upgrade your broker. Then you can convert your “A” application, so now an IS based service will be publishing the document. Your “B” application, which is still an old app, can still subscribe to the document and operate as normal.
You could also convert your subscribing app first, if you desire.
This is the “replacement method” outlined in webMethods_Integration_Platform_Upgrade_Guide_6.1.pdf, which you should have. There is also a “side by side” method in the document, which involves a territory. For most people, this is an unnecessary complication, but I recommend reading this, and then coming back to us with more detailed questions.
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