So I'm thinking of ditching Viranthian for a white dragon due to flavour. White dragon in the thorny wastes seems out of place, so I'm thinking of turning Tyr into a mature adult female white dragon (and Azar into a half-white dragon of course), then throwing the CR 11 blue dragon into the Ulwai fight (probably flying up to Ulwai soon after being attacked, probably while charging his breath weapon after a surprise attack).

Any balance issues seen with this? Does anyone else have experience with getting rid of the mis-matched Behir for another dragon?