One relatively simple way to beat the revenue cops on this kind of thing is easier if you collect miles/status in 2 programs. For example, I booked an advance purchase EWR-XXX, on CO, and midway thru needed to return 2 days early. CO would have made me upgrade to the Y fare ($600 extra) for the flight. I bought someone else's discount r/t promo fare XXX-JFK for $298, and ate half of each ticket.

Here, too, CO was matching the promo - and had seats open if I'd bought the r/t - but I didn't want to risk my silver status. So, I lost out on a couple of thousand miles, and they lost out on a couple of hundred $$. Since I've already cashed in those miles for a reward, unless the CO flight was sold out, who lost more?