Update Fund Center in Legacy Material Reservation which was Created before activating PSM

Hello Gurus,

With our client we are activating FM in existing SAP environment and now we have lot of open Material Reservations (MRs) which needs to be updated with Fund Center as we cannot derive it. In transaction MB22 all the fields in account assignments are greyed out, is there any possibility to make it in change mode or if we can update in MIGO. I can see in MIGO also there is no fund center field appearing for those MRs - Is there any possibility to change the MIGO screen.

Thanks in adavnce for help.

Regards

Rohit