THE PILOT WAS LANDING ON A FROZEN LAKE TO EXCHANGE THE MAIN LANDING GEAR WHEELS FOR SKI EQUIPMENT. THE INTENDED AIRPORT, A SEAPLANE BASE, HAD BEEN CLOSED DUE TO SLUSH AND SOFT ICE CONDITIONS. A NOTAM HAD BEEN ISSUED AND THE PILOT WAS ADVISED OF THE NOTAM PRIOR TO LANDING. DURING THE LANDING ROLL, THE AIRPLANE ENCOUNTERED SOFT ICE/SNOW AND NOSED OVER.