I certainly would not undertake a CPR swap with the express purpose of making money. There are many easier ways to "gussy up" a machine if flipping is your game. I would reserve this level of work/expense for something I truly loved.
Personally, if I had two machines side by side and one had an original playfield in nice condition and the other had a CPR I would probably pay a little bit more for the CPR (assuming it's a quality repro) but that's because I will take playability over originality on most titles.
A good example is my Joker Poker. The original playfield was nice but it had some cupped inserts that would cause occasional erratic ball movement. I still put in a CPR and now it plays so much smoother and the overall playing experience is greatly improved.
If I was a purist and hard-core collector, I might prefer to keep the machine original out of some misguided sense of romance
It comes down to who's buying. Just as beauty is in the eye of the beholder, so the value of certain mods/upgrades is in the eye of the buyer and will fluctuate greatly according to his/her preferences. LEDs are a great example of this.