5 Avoid attempting to upgrade from a VirusScan Enterprise patch level to an older version of ENS 10.x (for example, VirusScan Enterprise 8.8 Patch 9 to ENS 10.2). If the ENS version was released well before your VirusScan Enterprise patch level, a reboot is needed following the VirusScan Enterprise removal. The reboot can be avoided by ensuring your ENS release is newer than the VirusScan Enterprise patch level (for example, VirusScan Enterprise 8.8 Patch 9 to ENS 10.5.1).
However, VSE 8.8 P10 and ENS 10.5.3 were both released on Nov 13, 2017.. so they should "get along" but apparently not ? I will have to test when I can.