Issue in Production order "TECO"

Hello,

I tried to change a material type from HALB to FERT in MMAM, the thrown the message as follows,

"

Production orders exist for the material. This is not allowed if only

external procurement is defined for the new material type, or if the

new material type has a different account category reference than the

old material type."

and one production order was displayed and i checked the status of the production order, and was already "TECO", the status of the production order is as follows

REL Released

PRT Printed

PCNF Partially confirmed

DLV Delivered

GMPS Goods movement posted

MACM Material committed

PCC Product cost collector used

and production order is confirmed, Goods issued and delivered.

Now even after closing the production order also i am unable to change the material type, due to this production order. Where should i check and what should i do for changing the material type for the material. Kindly solve the issue

J.Balakrishnan