order unit of material

Hi Gurus,

We want to change the material to put an order unit, but we have an exsiting PO using the base unit of this material and

when we edit the material put an order unit the message come out "This order unit has been used already" I can't save

what I'm going to do? what will be the effect to the existing PO using teh base unit when we put an order unit?

hoping to your answer.

Thanks