Update GL Account number in account assignment PO

Dear Guru's,

I have a requirement to replace proposal gl account in PO after item is entered, this requirement it's because of for several document type they need to map the gl account into other account but didn't want to use config. It's frustating but this got to be done.

I have tried using EXIT_SAPMM06E_016 but looks like value changed after the exit.

If there is no other way I'm gonna use spot as my last resort, please help