core language – How to swap SetDelayed expressions?

In the rather unlikely case of you actually wanting to swap two definitions, e.g.

D1 := RandomVariate[UniformDistribution[{-1, 0}]];
D2 := RandomVariate[UniformDistribution[{0, 1}]];

You can do so temporarily

Module[{D1 = D2, D2 = D1}, Print[{D1, D2}]]
{0.863742,-0.710931}

If you’ve already used these definitions e.g.

print := Print[{D1, D2}]
print
{-0.349444,0.720738}

and would like to interchange them temporarily, you can use

Block[{D1 = D2, D2 = D1}, print]
{0.123198,-0.592611}

Needless to say, if you interchange them permanently, there is risk of becoming confused about how many times you’ve done it!