Yes, if EVC would allow the hosts to join the cluster, any VM which you vMotion to one of the hosts with the lower feature set would most likely crash. If you don't want to power off all VMs at the same time, you may create a new cluster with the new hosts, configure the required EVC mode and migrate the VMs one-by-one. Once all VMs are migrated move the hosts to the new cluster and delete the old one. Whether this is an option in your case, may depend on the cluster features used and any third party tools!?
André