To give you a head start, here are some of the KB links which I've collected having gone through the upgrade. Hope it help.
KB51569 - Supported platforms, environments, and operating systems for ePolicy Orchestrator
KB87673 - ePolicy Orchestrator 5.9.x Known Issues
KB51573 - Supported platforms, environments, and operating systems for McAfee Agent
KB51569 - Supported platforms, environments, and operating systems for ePolicy Orchestrator
KB87142 - ePolicy Orchestrator 5.9 Minimum Supported Extension Versions
KB66797 - Ports needed by ePolicy Orchestrator for communication through a firewall ()
KB90078- McAfee Agent 5.5.0 Compatible Products ()
PD26915 - ePolicy Orchestrator 5.9 Installation Guide - Rev. B
EDIT : Check out the Endpoint Security Expert Center which has lot more details.
Endpoint Security (ENS)
Cheers
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