Shelley-
You can use conditions to get these results.
On the OnScreenLoad event of the screen I would but a conditions to hide or disable Sales Rep if RecordID > 0 (Meaning its an existing customer) and to show or enable Sales Rep if RecordID < 0.
Depending on how you setup your screens you may have to add simliair actions to your Save & New buttons or Clear & New buttons.
-Michael