If any non-ELM devices remain out of contact such that you can migrate the agent remotely to ELM agent, your only option is to remove it.

If there are many hundreds of them scattered in many different nodes in your large device tree hierarchy, one quick approach to pick out all these devices in your MobiControl version is to use an alert rule with reasonably big timeout threshold for"out-of-contact" device event,  and choose action to relocate the associated device to a target virtual device-group/group.  After performing a quick visual check on the list of devices automatically relocated, you can easily delete all such devices by deleting the whole group.