The legal line has to do with title requirements for vehicles. There is no specific law that requires you to do anything in connection with the VIN when replacing parts, and you can swap VIN plates on vehicles, or destroy the plate – but that limits your rights with respect to the car. You need a certificate of title to operate or transfer (but not simply to own) a vehicle, per RCW 46.12.520. According to RCW 46.12.520, when applying for title, you have to include “A description of the vehicle, including make, model, vehicle identification number, type of body, and the odometer reading at the time of delivery of the vehicle”. This is trivially doable with an unmodified vehicle.
Once you have that Washington certificate of title, then apparently (based on a reading of RCW 46.16a), you simply have to keep registering it. There is normally no need for a further certificate of title, unless you try to transfer it. If a new certificate of title is required, you may be in trouble for crossing a line.
In case you need to apply for title (RCW 46.12.560) on a modified vehicle, the State Patrol may (probably will) perform a VIN inspection, since there would be a discrepancy in the description of the vehicle. The full list of inspection requirements is here and this document seems most relevant, being about “homemade vehicles”, as defined in WAC 308-56A-455. The core features of a homemade vehicle is that it is:
(a) A vehicle that has been structurally modified so that it does not
have the same appearance as a similar vehicle from the same
manufacturer;
(b) A vehicle that has been constructed entirely from homemade parts
and materials not obtained from other vehicles; or
(c) A vehicle that has been constructed by using major component parts
from one or more manufactured vehicles and cannot be identified as a
specific make and model.
In that case, you need notarized bills of sale or certificates of title for all of the major components – by RCW 46.80.010 this
includes at least each of the following vehicle parts: (a) Engines and
short blocks; (b) frame; (c) transmission and/or transfer case; (d)
cab; (e) door; (f) front or rear differential; (g) front or rear clip;
(h) quarter panel; (i) truck bed or box; (j) seat; (k) hood; (l)
bumper; (m) fender; and (n) airbag.
So replacing the seat may trigger the requirement for a VIN inspection by WSP. Of course there is the question of how they would know, but I'm only talking about the law.
There are various rules about the documentation that has to be provided, depending on the supplier: for example, if parts come from a private individual, the documentation requires everybody’s name, address, phone, description of parts, price, and the VIN of the original vehicle. If you can't do this, you can apply for ownership-in-doubt registration (no title) and 3 years later you can apply for title.
So, an oil change is okay, changing seats is mildly risky. While exchanging parts on a same make-and-model basis is unlikely to cause any problem, real problems could arise if there is ever a comparison between your license plate and vehicle description (if the police run your plates). If you truncate your Explorer into a Mini-Cooper, the mismatch between description and license will be noticeable, and they would have reason to think the vehicle was stolen.
