Jon,
Back in my Killian days, we took Rob Roy in Mt. Prospect, IL from 18 to 9 to add housing. Nearby Old Orchard downsized its 18 holes in length and par to accomodate some housing while maintaining 18 holes.
I have been commissioned to study each of those kinds of proposals. I would estimate that there are very few times the course could be made beter, although its more likely to get a better nine out of 18 at similar (but half) length than just a shorter 18.