Having done this a few times you have options. The long and short of iOS devices is similar to Highlander, you can only have the one.
1. In Apple Business Manager configure your Intune MDM connection and verify that everything is working as expected. Move ALL your devices in ABM from your current MD to Intune. Run a SYNC
2. In Intune, configure you Enrollment Profiles, Applicaiton Deployments, Compliance and Configuration Policies.
3. Configuration any User / Devices groups where you want to deploy applications.
4. Configure a Device Category with BYOD users will use (if you use option 1 below)
This is really simplified, but then you have 2 options:
1. Mass un-enroll devices from your current MDM solution. Have them install Company Portal from the App Store and then manually enroll. They choose a Device Category upon enrollment to get policies and applications.
2. You un-enroll devices from your current MDM, wipe all device, and then enroll the devices as if they were out of the box. You will use a Dynamic Device Group based on your Enrollment Profile to get the devices policies and lock their enrollment.
Hit me up if you have questions
