There are the following tables:

tblSuppliers
supplierName (PK)
invoicePaymentTime
(other fields)

tblGoods
goodsID (PK)
goodsName
supplierName (FK)

Each time I choose a "Supplier Name" I want "Invoice Payment Time" to be
filled with a corresponding value (30,60 days etc.).
My form uses a combo box for supplier name which asigns these values.

QUESTION:
In case "Invoice Payment Time" for a supplier changes (30 - 45 days) I want
a new value to be used for new records and don't modify existing ones.

The above described design doesn't allow this. How should it be improved?

Thank you for suggestions.

winXP
access 2003