Quick "On User Input" Question

Quick "On User Input" Question

I have a field that I want to to an action when the value has changed. Is there any way to know what the previous value of a field was?

Field Name: Carrier
On User Input
  1. input.Send_Email_to_Carrier = "Yes";

How do I test against the previous value? Hypothetically...
  1. if (previous_field_value.Carrier != input.Carrier) {//actions}

Any help would be great. Thanks!!!