Unfortunately, there is no way with version 6.0 that ensures the VM stays online.
One possible option is to move VMs from 6.0 to 7.0 is to power off the VM and do an offline migration, if they are both in connected vCenters.
Second option would be if you have shared datastore, to register on other hosts.
Third options is by restoring from backup.
Fourth option is by copying the files manually using WinSCP.
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