Why can't I reshape to any dimensions?

ArrayReshape[SparseArray`SparseArrayFlatten[m], {1, 8}]
(* or ArrayReshape[Flatten[m], {1, 8}] *)

SparseArray[<2>,{1,8}]

Normal @ %

{{1, 0, 0, 0, 0, 0, 0, 1}}

For the special case where the desired shape is a list with Length equal to Times@@Dimensions[m], you can also use SparseArray`SparseArrayFlatten:

SparseArray`SparseArrayFlatten[m]

SparseArray[<2>,{8}]

Normal @ %

{1, 0, 0, 0, 0, 0, 0, 1}

It seems one have to make SparseArray to Normal first to reshape it.

m=SparseArray[{i_,i_,i_}->1,{2,2,2}];
ArrayReshape[m,{1,8}]//Normal

But now

ArrayReshape[Normal@m,{1,8}]

Not sure if this is by design or not.

Comparing the timings for the approaches proposed by kglr and Nasser (modified):

$HistoryLength = 0;

m = SparseArray[{i_, i_, i_} -> 1., {16, 16, 16}];

(kglr = ArrayReshape[SparseArray`SparseArrayFlatten[m], #] & /@ {{1, 
      16^3}, {16^3, 1}, {128, 32}, {32, 128}}) // RepeatedTiming

(nasser = SparseArray@ArrayReshape[Normal@m, #] & /@ {{1, 16^3}, {16^3, 
      1}, {128, 32}, {32, 128}}) // RepeatedTiming

Verifying their equivalence

kglr === nasser

True