From my understanding, nLockTime is like a delay timer for transactions, and miners can't confirm them until the set time is reached.
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1. Can a transaction with nLockTime be replaced with a new one that has a different nLockTime value? What happens if it gets replaced?
2. If a transaction with nLockTime is stuck in the mempool because of low fees, can I use RBF to replace it with a new transaction that has a higher fee?
You can't correlate RBF with nLocktime since a non-final nLocktime prevents the transaction to be relayed by nodes,
While RBF is for transactions that are already broadcasted to the nodes' mempool.