Well her is my two cents worth. Upgrading parts or swapping parts comes down to what parts. Most manufacturers build equipment to a price point. The old Counterpoint SA-5000 preamp used a $7.00 volume control for a $4000 mfg. list priced unit. It sounds good but not the best that can be applied.
When I modify something I generally begin with the power supply. I use Nichicon caps there. They are very good sounding, not too expensive and they provide good results. I look at the other parts to see what is in there. Resistors like Mills, Aamtrans, Caddock to me always sound better in the correct position. I also like the sound of V-Caps.
Sometimes I feel the design is not the best and I may upgrade that also. That may require a little more skill. I also like to go with point-to-point wiring when I can.
Changing the house sound will happen but it depends on what you are looking for. More sound stage dimension? More details? Deeper Bass extension. Clarity, dynamics, slam, etc. Once you get a handle on what parts make those sound changes, you can get a better idea of what the unit will sound like.
This has nothing to do with the skill of the modifier, as anyone of you can swap out these parts. It is really not rocket science or very hard. Plus you can hear the differences and decide for your self if just swapping out parts works for you.
As far as the car analogy, yes you probably need more skill installing those modification but they are probably already designed and tested kits. For parts swapping, think of changing tire brands, performance tires, larger tires, to see how those change the performance of your car, installing larger brake calipers, things like that some of you can probably do on your own.
If you look at the Vandersteen speaker HP filters, they use WIMA capacitors and cheap resistors along with a dip switch that costs a dollar but the retail price for the pair is $900! I built my own and in my opinion they are a level up.
Just sayin'. Happy listening.