Change Order Unit in PR and PO on hold status

Dear gurus,

I have an issue from a user regarding changing Order Unit at PR and PO "on hold" status

Is that possible?

It's because user has key in Order Unit wrongly at PR as PC and now would like to change to LE (Performance Unit) at PO. System gives them errors as "Order unit LE not convertible into unit PC of requisition"

May I know how to solve this issue?

Thanks in advance!

Santthi