I'm also curious as to what is going to happen when (for example) 50% of nodes have enabled full RBF, 50% of nodes have not enabled full RBF, and I try to replace a transaction which is opted out of RBF. What about if I'm using a hot wallet and not my own node? Will I have to connect to different servers to find one which will relay my replacement transaction? And then presumably some block explorers will show the original transaction while some will show the replacement, and I'll have no idea which one will actually get mined until one of them is mined?
Wait, if I don't upgrade my full node to a newer version, I am risking getting an incomplete (wrong) version of mempool and, therefore, won't be able to verify independently everything that happens in the blockchain? It all sounds like, with this change, we are doing intentional hardfork forcing everyone to upgrade to modified software, albeit without changing consensus rules. That still may cause network partition and a decrease in decentralization because part of the nodes (pre-full RBF ones) will lose their status as being considered full nodes since they no longer will be able to conduct full verification of valid transactions.