I'm not sure if anything really would happen. It seems like a lot of people have had pretty good results/experiences with MGOCPM (I'm not sure how different it is from MGOCPS). How thick is your cap? It won't stop leaching, but having a cap should slow the rate at which substances can leach out of the soil and into the main tank water.
Other than that, I don't have much. How long since you did the changeover?