sparse arrays - directly change the background value of a SparseArray?

given a SparseArray

s = SparseArray[{1, 10} -> 1, {1, 10}, a]

I can extract the background value (see What are SparseArray Properties? How and when should they be used? )

s["Background"]

a

but is there a direct way to change it? We can of course convert to Normal form and create a new SparseArray

SparseArray[Normal[s] /. a -> b, Dimensions[s], b]

but note this will also change values that are explicitly a , and is undesirable in case the array is really big and really sparse then passing through the normal form is unwieldy.

Best I've come up with is to do FullForm and manually edit the third argument..

come up with an answer this, you can also answer this: Why does WeightedAdjacencyMatrix take the weight for absent edges to be zero?

Answer

You can change the background using replacement; here making the background "x":

s = SparseArray[{1, 10} -> 1, {1, 10}, a]

s /. (sa : SparseArray)[a_, b_, background_, d_] :> sa[a, b, "x", d]