Here's the way I see it:

Let's say Alice does not enable full RBF, but Bob does. Carol broadcasts a transaction which is not opted in to RBF, which propagates to both Alice and Bob's mempools. Carol then broadcasts a replacement transaction. Alice, running the old rules, rejects the replacement transaction as a double spend, while Bob, running the new rules, accepts the replacement transaction as an RBF. Alice therefore has the old transaction in her mempool, while Bob has the new transaction in his.

At some point, one of these two transactions will be mined in to a block. Which one is mined will depend on a variety of factors, such as how many nodes are running the old or the new rules, how much hashrate sees the original transaction or the new transaction, the fees both transactions pay, which miner is lucky enough to find the next block, and so on.

Once the next block is found, then all nodes, regardless of which rule set they are using, will be able to validate the transaction which was included in the block, and if necessary, remove the other one from their mempools. If the original transaction confirms, full RBF enabled nodes will eject the replacement, and if the replacement confirms, full RBF disabled nodes will eject the original.