A two-volume manga (2005-7) and six-episode OVA (2008, by Studio BEE TRAIN of Noir/Madlax fame).
When Princess Alita Forland flees her castle to escape a military coup, she bashes heads with the bounty hunter Falis -- which somehow causes them to switch bodies. Falis turns out to be a supreme fighter whatever body she's in, and at Alita's urging, she returns to the castle and puts down the coup. They have no way to switch back, so Falis-in-Alita's-body agrees to stick around until the prince returns to take over, while Alita-in-Falis's-body takes on the temporary identity of royal maidservant Milano Entolasia. Unfortunately, when the prince gets back, things only get worse...
Do Alita and Milano ever get unswapped? We'll never know -- the series ended too soon. All we know is that like most BEE TRAIN works, yuri fans love it.
Nobody here but us chickens!