So, OP, in your scenario, you have 1 transaction in the mempool, A, then you want to spend the change before confirmation, so you broadcast a new transaction, B, which replaces A.
> Because the size of the merged transaction is smaller than the original transactions, unless there is a considerable feerate bump, this rule isn't possible to observe.
I'm confused, the mempool only sees 1 transaction at a time, first A, then later B. " the original transactions", plural, should not exist in the mempool.
B's fee and rate needs to be larger than A's, but B will be greater than or equal to A anyway. So, just increasing the fee rate will cause a larger fee anyway.