Nimble Obstructionist > Brazen Borrower: A shoe-in swap. I've been high on Obstructionist for a while, but Brazen Borrower offers much more flexibility and is a 2-for-1 rather than making you choose.
Remorseful Cleric > Charming Prince: Remorseful Cleric wasn't in for very long, but it was a solid role-player. Charming Prince is good in a multitude of decks - all three modes can be good in midrange, control benefits from the gain life and the scry, and aggro now has a 2-power creature that can fix future draws. Bonus points for being incidental flicker support.
Phyrexian Arena > Murderous Rider: Arena has been seeing less and less play as a relic from its time. Rider is the second Hero's Downfall I needed - instant speed, low drawback, and a guaranteed creature on the back half. Rider as a creature is an unassuming 2/3 but can quickly flip a board.
Vivien's Arkbow > Once Upon a Time: Arkbow had consistently underpreformed after replacing Birthing Pod. Once Upon a Time is an exciting new card that I am slotting into a test slot to see if it pans out.