The only way to migrate from ZHA integration to Zigbee2MQTT today is to re-pair every single device.
You can make that process much easier on yourself though by buying a second Zigbee Coordinator radio adapter (Texas Instruments CC2652P based), flashing that with latest firmware before setup a new Zigbee network on it in Zigbee2MQTT, as then you can migrate one Zigbee device at a time by removing it from the ZHA integration which should automatically factory reset the device and put it into pairing-mode so that you can directly enable joining in Zigbee without having to run around manually resetting all devices.
If you absolutley want to re-use your SLZB-06M as Zigbee Coordinator in Zigbee2MQTT then suggest first migrate ZHA to a other less expensive Zigbee Coordinator to free up the SLZB-06M →
Then you a repurpose your “old” Zigbee Coordinator radio adapter you migrate from as a dedicated stand-alone Zigbee Router device by reconfiguring it as a Zigbee Router or reflashing with Zigbee Router firmware (which will depend on type), then powering it via a USB-changer for permanentpower. Tip is that Silicon Labs EFR32MG21 based USB adapters (like fir example the Sonoff ZBDongle-E) make a slightly better edicated stand-alone Zigbee Router device than a Texas Instruments CC2652P based USB adapter.
See more information and related tips here: