Windows Server 2016 and 2019 are licensed per CPU Core. (no HT pure cores)
So you need at least the number of licences as you got cores in the system you want to install it.
Then as for Hyper V. One correct set of licences of the Standart edtition allows you 2 installations. But no cross version... when you get 2019 you are allowed to setup 2x 2019 on the system, same with 2016.
so one 2019/2016 (depending on the licence you bought) and another in the hyper V on that machine.
Downgrading, afaik is only for enterprise volume/select contracts with software assurance.