Danger of using old tax code in purchase order

Materials management, Finance and controlling modules are active in

system.we have created new tax codes at the end of Dec 2007 in system

as per the legal requirement.Also condition types are maintianed with

validity period. We have updated existing purchase orders (which are

going to be continued in year 2008) with new tax codes.

But now if by mistake while creating purchase order, user picks up old

tax code and saves PO, system does not give any error.

Is there any way by which we can restrict using of old tax codes at the

time of creating / changing Purchase orders?

Thank you for your time.