Based on the % change you want to see, you'll need to do something slightly different whenever A=0 or A<0. So, using your examples of A for original value and B for the new value: if A is 0 determine if your % change should be 0 or +/-100 based on the value of B. If A is not 0, handle negative values of A by taking the absolute value in the denominator; otherwise use the normal % change formula:
=IIF(Fields!A.Value=0,
IIF(Fields!B.Value=0, 0, Fields!B.Value/Abs(Fields!B.Value)),
IIF(Fields!A.Value<0, (Fields!B.Value-Fields!A.Value)/Abs(Fields!A.Value), (Fields!B.Value-Fields!A.Value)/Fields!A.Value))
Seems to work for your provided values and for a few others I made up: