I would create a formula and then display the formula on the report. This way you don't have to override existing data, you are just displaying the new data in a different field.

if A>10 and B=10 then
    'Good'
else
    CStr(B);

The thing you have realize is that the If statement has to return the same data type. So that is why I converted B into a string instead of leaving it as a number.

Are you using Crystal syntax? I just typed it into a dummy report and it works fine. Did you type it in exactly as it is here? Maybe you added a semi-colon after the 'Good' line? Copy and paste it here if you still get the error.