How to change the Currency Rate Type.

Hi,
When we make the foreign currency transaction we enter the Exchange Rate in AR Transactions, This Exchange Rate type comes as "Corporate" by default in AR Transaction form, We need to change this Currency Type as "User" in AR Transaction form.  When i make the Transaction the Currency Rate type should shows as 'User" by default in AR Transaction form.

How to set this Currency rate type as default?

Instance : R12
Module   : Receivables

Thanks
Mohan