I am redoing my attic, which used to be finished with water-damaged, lath board plaster. The plaster is wrecked in places so my plan is to rip it out and hang blueboard. However, the knee walls are in good shape - I was wondering if it would make sense to leave them and just replaster? It would save a little in materials, and a significant amount of time both in the demolition and rehanging. It seems a little half-assed, but I can't really see a downside. I even thought of hanging blueboard right over the knee-walls, but I don't see why it would be necessary.
I assume you are using blueboard because you are plastering??? If there are large gaps in the old plaster, I would fill them with drywall mud, then go over with drywall, blueboard, etc. Make sure you screw into the studs, not just the lath.