Words like irreversible are enough to turn a sysadmin's hair grey and fall out in large clumps and unfortunately that is a word that applies to those who upgrade to VSphere 5.5 while running VMWare 5.1.  The good news is that if you have not upgraded yet, you can now get the VMWare 5.5 update which is compatible with VSphere 5.5 and will ensure you do not have to install VMWare 5.1 again from scratch and rebuild everything.  Follow the knowledge base links from The Register to update or to attempt to save your replications if you upgraded prematurely.

"But as explained in this knowledge-base article, “if you have Automatic Check and Install updates selected the vSphere Replication appliance will automatically upgrade to version 5.5”. If that happens on an existing vSphere 5.1 installation, very bad things will happen."

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