Hi,
I hope below steps will helpful for migration.
1. Take a full backup of your old router.
2. Create a Logical and physical network diagram for reference.
3. Note down all policies, route, objects, special permission etc.
4. Note down changes, if there will any in new device or topology.
5. Make new device ready with all configuration.
6. Choose any testing method between Pilot and Prototype. (According to your downtime, resources availability)
7. finish the testing (one by one) and shut down the old device.
But If you can upload a backup file into your new device then it is highly recommended.
Regards,
Deepak Kumar
NSE4