The Spirit Channeler discipline allows psychics to change form to another form of your type and subtype—but that the new form cannot change your type or subtype.
Metamorphosis (Su):
By meditating for 1 hour, you can trigger a slow, physical metamorphosis that takes 24 hours to complete (during which you do not gain the benefits of this new form). This ability cannot change your type or subtype, grant new abilities or natural weapons, or change your size category, but otherwise allows you to transform your appearance into anything possible for creatures of your type and subtype. You gain a +1 enhancement bonus to one physical ability score of your choice (Strength, Dexterity, or Constitution) while in this altered form. This bonus increases by 1 for every 5 psychic levels you have. Your shape and ability score bonus persist until you assume a new form or your current altered form ends. When meditating to begin your metamorphosis, you can spend 1 point from your phrenic pool either to change your size category to Small or Medium, or to gain a new ability from those provided by the alter self spell. If you do so, your altered form lasts only 1 hour per level.
By meditating for 1 hour, you can trigger a slow, physical metamorphosis that takes 24 hours to complete (during which you do not gain the benefits of this new form). This ability cannot change your type or subtype, grant new abilities or natural weapons, or change your size category, but otherwise allows you to transform your appearance into anything possible for creatures of your type and subtype. You gain a +1 enhancement bonus to one physical ability score of your choice (Strength, Dexterity, or Constitution) while in this altered form. This bonus increases by 1 for every 5 psychic levels you have. Your shape and ability score bonus persist until you assume a new form or your current altered form ends. When meditating to begin your metamorphosis, you can spend 1 point from your phrenic pool either to change your size category to Small or Medium, or to gain a new ability from those provided by the alter self spell. If you do so, your altered form lasts only 1 hour per level.
I wonder what this means for Aasimars (type: outsider/subtype: native).
It's clear they could take the appearance of tieflings, sylphs, undine, etc.
It's slightly less clear whether you could take the form of, say, a rakshasa, though my sense is 'no'.
I also wonder how this relates to aasimar with the Scion of Humanity variant.
Scion of Humanity:
Some aasimars’ heavenly ancestry is extremely distant. An aasimar with this racial trait counts as an outsider (native) and a humanoid (human) for any effect related to race, including feat prerequisites and spells that affect humanoids. She can pass for human without using the Disguise skill. This racial trait replaces the Celestial language and alters the native subtype.
Some aasimars’ heavenly ancestry is extremely distant. An aasimar with this racial trait counts as an outsider (native) and a humanoid (human) for any effect related to race, including feat prerequisites and spells that affect humanoids. She can pass for human without using the Disguise skill. This racial trait replaces the Celestial language and alters the native subtype.
My initial reading was that you could assume the form or either human subtype or native subtype, but RAW this seems to imply only those with both humanoid/human AND outside/native types and subtypes.