Management or Gateway?
If it's a management and is a CheckPoint device which was upgraded from 80.20 to 80.30, during the upgrade procedure it's creating a Snapshot. You can see it in the Web GUI of the management under Snapshots and you can revert to it. If it was firstly installed with 80.30 I'm not aware of such downgrade procedure. It the policy was extracted from another management with lower version, you can preinstall the new device with 80.20 and play with the policy and import.
If it's a gateway, just run show configuration and take the output and preinstall the device with 80.20 and import the configuration.
It it's a standalone installation you should check both procedures described above.