Hello! Since you say "VLAN implementation" Am i to assume you have used unmanaged switches prior? If so i would as a step one set the ports for KVM on untagged/Native VLAN ports so that the switch change is transparent to ovirt. Then you can go ahead and set up new VLAN tagged virtual interfaces that you can migrate the management functions and so on to later once everything is as you wish.
This way you do not need to reconfigure all of the features you have in ovirt initially.
Other concerns would be regarding your storage nics and so on. Especially if you run hyper converged as you will be isolating the nodes gluster traffic. Do you have multiple nics bonded for redundancy in active/passive? If you do then you can move one half of the nics as long as the old switch and the new switch have a link where the old switch runs on the temporary untagged/native VLAN that you move the ovirt cluster to.
With the limited info you have provided i think this is just about everything i can think of.
Good luck!