This isn't always true. You have full control over the inputs of an unconfirmed transaction only if the transaction has been marked as RBF.
If your transaction hasn't been marked as RBF, it's very unlikely that you succeed in replacing that with a new transaction spending same input(s).
Nodes usually reject any transaction spending UTXOs that have already been spent in a non-RBF transaction, even if the original transaction is still unconfirmed and you have paid a very higher fee for the new transaction.
Pardon my ignorance but first, you said the double-spend transaction relays "only if" it was marked as RBF beforehand, but then you said that "if" it was marked as RBF, it is very "unlikely" it will relay and that nodes "usually" reject such a transaction. Could you elaborate on that? Are there special circumstances in which it is possible to get around existing network restrictions and get your double-spend transaction confirmed before the initial one is confirmed? Given that a double-spend transaction is still valid to the network and also has significantly higher fees, miners will want to include it. The only problem I see is that miners won't know that such a transaction even exists because the nodes of the network won't tell them it exists. What if a person who wants his double spending to succeed merely gets in contact with mining pool operators and asks them to include this transaction in a candidate block instead of the first one?