The upgrade guide should be considered the final word in the matter, though it may "work" on previous versions, it won't have been fully tested and through the formal QA process.
Prof Services can probably help you if you want to try upgrading from an "unsupported" version - Support won't be able to though.
however still confuse as one side McAfee's upgrade guide is not talking about 5.1.x clients upgradation and other side on below McAfee Link says that one can upgrade from 5.1.7
"Can I upgrade EEPC 5.x to EEPC 6.0?
You can migrate to EEPC 6.0 from the following versions:
- EEPC 5.1.7, EEPC 5.1.8, and EEPC 5.1.9
- EEPC 5.2.x or later "
My question is if I have mixed environment of (5.1.9 & 5.2.5) Can I go for migration v6.1.1 ? or I have to upgrade my all client from 5.1.9 to 5.2.x first?
As far as I can see, the KC and upgrade guide agree - 5.1.7+ is supported?