First we increased the depth of the amplitude modulation. This increased the reception capability by a few db - but didn't solve the interference problem from Rugby. An East coast station operating at 40 kHz at the Redstone Arsenal in Huntsville, Alabama was attempted but didn't get completed. So then we took a new approach. We added phase modulation to the existing broadcast while keeping the existing AM modulation intact so that all existing devices continued to work.
The new modulation modifications became permanent in October of 2012. The changes improved reception by 14 db with several more db possible using extended reception techniques.
New products are now coming out using this much improved technique. Time will tell how widely this gets used!