Only way to use 8 in parallel is to buy a new extra key. As for downgrading existing key, this would give you a new different key and you would need to re-enter it, using the old invalid keys left by this process would be a big no no

Check the version of your ESXi hosts if they are compatible to vCenter version 8 then upgrade your vCenter to version 8. By doing this you can also manage the ESXi hosts having the latest version.

Regards,

Sachchidanand

If you have 1 vCenter license you can run only run on licensed instance at whichever version. However if you don't license the new one you can run it with full functionality on the 90 day eval key. Is this long enough for you to decide whether v8 works for you? Don't vCenter v8 will support both v7 & v8 ESXi hosts.

I know that, but key was for more than 1 instance. So I divided upgraded license and then downgrade one of two new licenses. Hope it's compliant, support didn't get me undoubtedly answer. 

If you had two licences on one key and then divided & downgraded so one license is v7 and one is v8 that's not a problem.

If you have two licenses then why you want to downgrade and upgrade again the same key. Use both the keys separately one for v7 and one for v8.

Regards,

Sachchidanand

It's because one key is for two instances.

most probably you are using eval icensing for vcs 8. you have to buy new license for upgrading to major version