Transferring DownValues

You can make use of the undocumented Language`ExtendedDefinition function that underlies the Wolfram Cloud. Suppose:

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

Then:

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

Finally:

DownValues[cc]

{HoldPattern[cc[1, 2]] :> 13, HoldPattern[cc[1, 9]] :> 87, HoldPattern[cc[2, 6]] :> 8, HoldPattern[cc[3, 7]] :> 4}

DownValues[cc] = DownValues[cctemp] /. cctemp -> cc;

cc @@@ {{1, 2}, {2, 6}, {3, 7}, {1, 9}}

{13, 8, 4, 87}

If cc has some assignments that are not overridden by the assignments in cctemp you can use

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

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