Inserting a new OES 2018 into existing OES2015's eDirectory

I wanto to install a new OES 2018 on an existing eDirectory hosted on OES 2015 SP1.
Does eDirectory 9.x is a requisite (I'm not sure about), so I've to upgrade to it  (OES 2015 now have eDyrectory 8.8 SP8)?
Any more suggestions?

Thanks,

Fabrizio