Provided that you're not using a distributed switch, all you have to do is disconnect and remove them from the first vCenter and add them to your vCenter 7 instance. If you are using a vDS then the process is more complex as you first have to migrate to standard switches. It's more complex still if you're using vSAN, NSX, or some other solutions. Also, keep in mind that if you're using a backup or monitoring solution, as soon as you swing those ESXi hosts to the new vCenter, those tools will see the running VMs as essentially "new" virtual machines. Although the process to swing hosts is easy, there are lots of things to consider.
Please don't mind me asking, but why didn't you simply upgrade the existing vCenter Server?
Whether it's possible to connect the ESXi 6.5 hosts to the new vCenter Server without downtime depends on some things, like advanced features that are in use (e.g. Distributed Switches).
If no advanced features are in place, it should basically be possible to add the hosts to the new vCenter Server (on the Datacenter level), and move them to the cluster in a second step.
Please consider to disable HA, and set DRS to Manual on the old and new cluster before starting to migrate the host.
Since I didn't do this myself with that specific versions, I recommend that you start with one host with only a few VMs, so that in case it doesn't work as expected you can shut down those VMs.
Thank you for Your response.
We are not using vDS on those servers, neither NSX or vSAN, so it should be good to og to disconnect and remove the ESXi hosts from the existing vCenter and add them to the vCenter 7.0.0a.
Yes I know have to create new Veeam backup jobs through the new vCenter.
Thank you for your response and advice!
I understand why you are asking why we don`t upgrade our existing vCenter. That is because we have two separate VMware environments with two separate vCenters. Now we are going to merge these VMware environments together and use one vCenter based on vCenter 7.0.0a and ESXi 7.0 hosts only.
The tre ESXi hosts will be shutdown after migration after we have migrated their VMs into new ESXi 7 hosts in vCenter 7.
We are not using distributed switches, NSX or vSAN on these servers.
Do I have to disconnect the ESXi host before I remove it from the Inventory, because I othervise got a message that i have put the ESXi host in maintenance mode befor I can remove from inventory?
I currently do not have an environment to test this, but IIRC disconnect->remove doesn't require Maintenance mode.
OK, then I have to try the disconnect and remove the ESXi host from Inventory-solution with not too many VMs on ....