cc[1, 2] = 1;
cc[2, 6] = 12;
cc[3, 7] = 3;

cctemp[1, 2] = 13;
cctemp[2, 6] = 8;
cctemp[3, 7] = 4;
cctemp[1, 9] = 87;

DownValues[cc] = DownValues[cctemp] /. cctemp -> cc;
cc[1, 9]

cc[1, 2]

DownValues[cc] =  DeleteDuplicatesBy[
   Join[DownValues[cctemp], DownValues[cc]] /. cctemp -> cc, #[[1,1]]&]

cctemp[1,2] = 13;
cctemp[2,6] = 8;
cctemp[3,7] = 4;
cctemp[1,9] = 87;

Language`ExtendedDefiniition[cc] = Language`ExtendedDefinition[cctemp] /. cctemp -> cc;

DownValues[cc]