Not exactly. Any change in the amount of FWK cores will cause the cluster to break. A member with the lowest amount of FWKs will become active. Mind, you will need to reboot a member every time you change cores assignment. To re-align, you will have to reboot the second member as well, after setting the same amount of cores.
Two reboots, two downtimes without synchronization. Hence, a service window.
In resume, it does not matter if you increase or reduce amount of SNDs. You will break cluster either way. The only reasonable alternative is to migrate from open server to Check Point appliances. There, with any supported version, you will not need to redefine SNDs and FWKs, because Danymic balancing with take care of that on the fly, without breaking a cluster, in an automated and transparent manner, not requiring any manual action from you.
More information about Dynamic Balancing is available in sk164155
Also, an important note. R80.40 support ended in April, you need to move to R81.20 ASAP.