Yeah I don't know if it's illegal, but pretty much every plane is needed somewhere else if there is a scheduled flight for it regardless of load.

Subbing out any one plane for a light load is probably not worth the hassle. It's not like the airline doesn't know well in advance the load is going to be light. What is MUCH more likely to happen would be that flight gets axed 3ish months out. DL has historical data to know when people book certain routes. So at 3 months out of a flight is only 50% as full as they think it should be, they might reduce the schedule for that day as a sked change, and they'll move the handful of people onto other flights, especially if there are multiple segments that day.

For example if DL flies 5 flights a day from X to Y, and one of the flights is well under where it's projected, they may reduce the flights that day to 4 especially if it's one of the 3 middle flights. OR they'll do an equipment swap and sub a smaller plane.

But once you get to DoD there is a virtual certainty that flight goes unless there is weather or a mechanical...and DL is highly unlikely to game it saying it's a mechanical when really it's just a light load. DL cares very much about their DOT stats.