Updation of Assignment number in FBL3n

Hi,

While posting user has entered assignment number which is there in voucher but when i open fbl3n, in report Assignment field is showing as "00000".

Actual in FS00, control data earlier sort key was Personnel number which i have change to Allocation number. And now the Assignment field is showing the data.

Please let me know what can be done for those previous entry which is still showing as "00000". There are many entries.