It should stay in your originally booked fare class, but a couple of times a year I get credit for full-fare in whatever class I end up sitting in. Most recently this happened to me last week, trying to get the hell out of CDG after it received two whole inches of snow.
One necessary, but not sufficient, condition for this is that an agent modifies my itinerary manually. Some examples include a reroute through a different city (or on a nonstop instead of connecting flight) during irrops, or a serendipitous double upgrade on a 3-class plane, or an eqp swap throwing off everyone's seat assignments.
Needless to say, if for some reason you get credit for fewer miles than the fare bucket entitles you to, you'd want to call MP to fix it.