Otherworldly feat + templates.

[Yogibear41]

Can a character begin the game with a template that requires humanoid type, but then change his type to outsider at 1st level via the otherworldly feat, while legally keeping the template/its benefits?

[pabelfly]

Can a character begin the game with a template that requires humanoid type, but then changes his type to outsider at 1st level via the otherworldly feat?

I'd say so. Character building order has race first, then templates applied, and some time after that, you pick your first level feat.

[Saintheart]

Can a character begin the game with a template that requires humanoid type, but then change his type to outsider at 1st level via the otherworldly feat, while legally keeping the template/its benefits?

So long as the template doesn't change the character's original type, then I think you could get away with this by RAW.

But if I wanted to be a smart@$$ DM, I'd reply that "the moment you apply a template to a type, the creature gains the [Augmented] subtype. For humanoids, your race is your subtype. You have to be of the specific racial subtype and nothing else in order to take the feat. No, I don't care that you have both the [Augmented] and the racial subtype, you have to have only the one to qualify for Otherworldly."

This would be even more clear if the template changed the character's original type away from humanoid, because doing so means the creature's type is NewType [Augmented], not humanoid, and the creature doesn't retain both the traits and features of the original type - it has the features of the original type but the traits of the NewType.