Hi,
I gonna explain to you the best way to do this migration:
1. First export the master key from the old unit to the new one, using f5mku -K to read the master key in the old device and then using f5mku -r "old master key" in the new device.
2. Download a backup on your device and open it with 7zip, open in configuration the bigip_base.conf and replace the management IP of all devices with the new one, save the change in the text editor, and re-build the backup with 7ZIP " this is automatic after changing a file"
3. Load the backup in the new device excluding the license and platform check using the cli, tmsh load sys ucs no-license no-platform-check
4.Solve all the possible conflics review tha fails with tmsh load sys config, it could be name of the interfaces, trunks, etc.
5.After the configuration in the new device are ok, download using winscp the files Bigip.conf and Bigip_base.conf, in the first one you could edit all the names for the services, and on the second one, you could edit all the network parameters.
With this model you could migrate all the topics from the all device to the new one, and you can avoid many mistakes in the process.
Hope it´s work.