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: P&O Oceana December 2016
Next: NCL Jade August 2017
Then: P&O Ventura December 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(4), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey, CMV Magellan