Are augmentations permanent?

I've just received my first praxis point and am now fretting over what to spend it on.

Are my choices here permanent or can I unspend a reallocate points to different augmentations later in the game?

Augmentations are permanent. There is no way to re-spend your praxis points short of starting a new playthrough. (But let's be honest: why wouldn't you want a 2nd playthrough!)

Technically, according to the game, you start play with every augment, and praxis points only represent knowledge that helps you "turn on" the various augmentations*. Unlike in the original game, there is no exclusive choice when it comes to augmentations. You can get as many as you like -- as long as you have the praxis to pay for it.

*According to a scene early in the game, part of the post-augmentation period is what allows the brain to fully integrate with the augmentations. That is to say, the augments kick in automatically, after they interact with the brain for enough time.

In a word, that abstraction is the reason you're gaining "XP" and "levels". Each level (and the praxis point it brings) represents Jensen having spent enough time to "naturally" unlock the dormant augmentation. Praxis Kits, are objects that let you side-step the "natural" process, and directly download information to tell the brain how to use it.