Lots of admissions on here
It's the real world though I suppose, you'd have to be pretty naive to not think about this sort of thing when buying an RS.
I see what your saying about the car and keeping the original 32k ID, if you are planning on keeping it why not, but it's a lot of hassle and work to go through to have it technically as a low spec car that won't be worth very much, but if you get the enjoyment out of it why not.
BTW I am SURE you can only do the 70%/30% idea and keep the ID of the old car if it is a new shell, and has not been previously registered.