It's important to use the VMware Product Interoperability Matrices whenever you are planning an upgrade. They have a note for vSphere 6.7.
VMware Product Interoperability Matrices
You will have to upgrade all or some of the hosts to ESXi 6.7. VMware outline the process in the Horizon documentation from 7.5 on.
Upgrade Instant-Clone Desktop Pools
Note:
If you upgrade vCenter Server to vSphere 6.7, then all or some of the ESXi hosts in the cluster must be upgraded to vSphere 6.7. Else, the instant-clone desktop pools might not work correctly.