The tool always replace the function call tag, in the expression, by its result. Returning nothing is not a condition, and can't be, to not change the expression result or discard the use of it.
The best way to accomplish this is by passing also the effective current field value as parameter. Let's say you are using the TestSwitch in the Title field. You may write the expression as: [TestSwitch](2,[T])
And change the script to:
function TestSwitch() {
var optionID = CurrentField.Value.substr(0, 1);
switch (optionID) {
case '1':
{
return 1 // change values to 1
}
break;
default:
return CurrentField.Value.substr(2); //returns field original value
}
}