Hi,
As new message output type ZNEU working well on PO creation & on PO change and you do not want old message output type NEU.
Use t.code:NACE,Application EF--> PO Output type --- > Select your old PO output type NEU and in General data TAB, DO NOT SELECT check box of "Access to condition" and save.
Also if message condition record maintained for message output type NEU and deletemessage condition record maintained for old message output type NEU in t.code:MN05.
But you should maintain the message condition record for new message output type ZNEU with all PO document type!
Now go for change of existing PO and see the difference!
Regards,
Biju K