Earlier this year I did a monthlong Roadtrip with 4 teenagers around northeastern Canada and USA using only points. I repeatedly fiddled around with the dates and rooms, and for personal reasons needed to be able to fly back to the UK at any time, I think none of the reward bookings I made in January actually became part of the holiday, I booked, rebooked and fiddled around so much!
THe great advantage of all points redemptions is that they are fully cancellable till day before. It's a great feature!
IN short, cancel and you get your points back!
I book everything online. When I have to cancel a stay, I just go online and cancel the reservation. My points are usually back in my account immediately. If it doesn't show, I just log out and log back in. It's put back in once I re-log in. Marriott is very good about this.