Last I checked, Check Point does not support clusters with different member hardware for any length of time, no matter how small. The official answer is you MUST take an outage to swap.
That said, this is one of the combinations which could actually work, since the 6900 uses an i9-9900KF (8c16t) and the 9300 uses an i5-13400E (10c16t), so the OS sees 16 cores on both. Both run 14 workers by default. The big concern is the 9300 wants to have UPPAK enabled, while the 6900 does not. I'm not 100% sure that configuration can successfully sync.