Ways to transfer, project, split, and generally manipulate consciousness/soul?

[RoSothian]

I'm looking for a way to have your spirit inside something else to gain powers specific to that something else, while having your body still functioning normally. Like putting it into the biggest tree of a forest, then gaining control over the lesser creatures of that forest.

[Doctor Despair]

If you want to control a Treant, you could dominate it/mindrape/etc. it, and then control its animated trees that way. If you specifically want your spirit to be in it, you could Magic Jar yourself in, though that puts the Treant in your body. I think the mechanic you're looking for might be in the psionic Thrallherd class, perhaps?

[ATHATH]

Lichdom is the classic way to do this.

Bodak Birth might also align with what you're thinking of.

[MisterKaws]

Find a way to persist Fission, then use any form of mind switch.

[Telonius]

Fiend of Possession (from the Fiend Folio) can do this. It's for [Evil] Outsiders only, but a Tiefling (or any other Outsider) with a Ritual of Alignment (from Savage Species) could pull off the requirement.

[elonin]

doctor despair,

Magic jar leaves the victim in the crystal not in your body

[ATHATH]

Possess Animal and the Mindswitch line might also work.

The Soul Eater prestige class can transform into what it kills, IIRC.

[Jack_Simth]

Chain Spell Magic Jar: Posses several things at once. Apply other metamagic to taste, such as twin and repeat.

[MaxiDuRaritry]

Chain Spell Magic Jar: Posses several things at once. Apply other metamagic to taste, such as twin and repeat.Twin and Repeat only cause the spell to be cast again on things you've already cast it on. Helpful if you missed some; not so much on the ones you didn't.

How about Ocular Spell, Split Ray, and Chain? Twice as many possessions at once.