I presume that you already have the Princess return dates, so are fairly tightly bound on the TransAt options by that.
It might be worth talking to an agent and asking that "as they are both part of the Carnival group, can they try for a special price". They will often do it for follow-on cruises on the same ship, so who knows they may be happy to help, especially with the current high availability on the ships.
Good luck & hope you have a great time.
Last Cruise: P&O Britannia March 2015
Next: Minerva April 2015
Then: Royal Princess August 2015
and : Queen Victoria January 2016
Golden, Grand(2), Caribbean, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(6), QE, Caronia(2), Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey