Can I customize my new iPhone 5s with older iPhone 5 parts?

Tom, I wouldn't quite say that. Apple does use a lot of standard off the shelf parts (like the little SMT stuff). All smartphone makers do their own thing with very little that is transferable to the next generation of device, Apple is no different here. I think the fact the general shape (and look) of one generation to the next of Apples products is so similar that it's assumed many of the parts would be transferable. I'm sure Apple doesn't do this to !@#$ us off, more likely its about maximizing the space so everything fits inside.

I mean that parts are not interchangeable even between apple devices. Most other manufacturers use standard packages on PM, audio, wifi and even CPUs. All these are custom packages on apple devices and unlikely to be interchangeable. Of course small SMT parts are standard, no point wasting money for almost no benefits.

mmm... Soc (CPU): no one is swapping them out, more likely going to do the whole logic board at that point. Audio: a discrete DAC chip is sliding away due to space in all of the current Gen of smartphones. Battery: well that gets back to space limits. I didn't say Apple didn't make custom components, but if they had used off the shelf SoC they wouldn't have a 64bit unit in the 5s. I think of Apple products are like a Bugatti race car pushing the limits of the technology, but unlike the Bugatti we all can get one. You won't get that using 'off the shelf' stuff.

I suppose 5s parts won't work in a 5 ?

No, compatibility is not one way Vs the other for any version of the iPhone.