Apparently there is a Wikipedia page dedicated to Titanic Conspiracy theories, who knew!

In regards to swapping the Olympic with the Titanic, this section summarizes:

British historian Gareth Russell, for his part, calls the theory "so painfully ridiculous that one can only lament the thousands of trees which lost their lives to provide the paper on which it has been articulated." He notes that, "since the sister ships had significant interior architectural and design differences, switching them secretly in a week would be nearly impossible from a practical standpoint. A switch would also not be economically worthwhile, since the ship's owners could have simply damaged the ship while docked (for instance, by setting a fire) and collected the insurance money from that 'accident', which would have been far less severe, and infinitely less stupid, than sailing her out into the middle of the Atlantic with thousands of people, and their luggage, on board, and ramming her into an iceberg".