Functional Area cannot update when posting

Hi,

Now I am confused about functional area when posting document.

Functional area derived from master data of Cost center.

cost center 1000 -> functional area 2000

cost center 1001 -> functional area 2100

And no functional area derived from account and cost element, also no substitution.

When use F-02 to posting document, first I input one line item with cost center 1000, then the system derived functional area 2000. The document has not been posted. Now i want to change the cost center to 1001, but after change the cost center, the FA still with 2000, not changed to 2100 automatically. so it is not correct.

My version is ECC6.0. I did the same thing in IDES ecc6, but the system works fine, after i change the cost center,the fuctional area also changed to 2100.

I compared two systems,but i do not know why, anyone can help me? thank you!

how did the system update data when we input the document?