Not able to change delivery address in PO having account assignment cat M

Hi All

We have a scenario where we are creating SO then a PR is getting created from this SO and finally buyer is converting Preq in to PO (account assignment is M as it is against a customer order), now by default the delivery address in PO is coming to that of customer address , but we want to change this delivery address to that of a location where some assembly work will be done. But system is not allowing us to change the delivery address, as soon as we change the address system overwrites it with the original customer address.

Please provide your valuable inputs.

regards

Yogesh