Officially:
Solars can also become Primordials if they really want to (they typically don't).
Semi-officially:
It's also hinted at in Dreams of the First Age that Terrestrials that reach Essence 10 and buy "Transcendent Gaian Harmony" five times might reach some kind of never-before-seen apotheosis into a higher form of being.
Psuedo-officially:
There exists a legendary quote. Somewhere. Somewhere, there is a developer that says definitively that every kind of Exalted gets the option to transform into something decidedly inhuman at some point. Everyone gets the chance to transcend to a higher state.
I make pilgrimages into the internet to find this quote almost daily.