Here are my suggestions.
1) Upgrade existing appliances to R81.20, then simply follow below link
https://community.checkpoint.com/t5/Security-Gateways/Replace-Upgrade-Cluster/m-p/157228#M27268
OR
option 2) Get config of current appliances, you can simply run from expert -> clish -c "show configuration" > /var/log/currentconfig.txt , then get that file and copy bits and pieces over to new firewalls. Now, this might be a bit tricky since interfaces will never match, so it may take some time.
Personally, this is just me, I would do option 1, but again, thats how I had always done it.
Best,
Andy