Obtaining an existing dropdown list value in a field before it is changed

Obtaining an existing dropdown list value in a field before it is changed

Hi

Maybe simple question, i have a drop down field with Sales1 / Sales2 / Sales3 as the choices
if the field is changed i want the existing value to be written to a form "Original Sales Person" and the new selection to be written as "New Sales Person"

Basically i am trying to record any changes regarding who is handling the sale
The problem is that within a field onupdate script only the new value is available

I tried to get the original value when the form loads or at another stage but then i have the problem that the variable stored on form load is not available in a field update script

Any suggestions

Thanks