Going from 4 FM areas to 1 FM area

Dear experts,

I work for a customer who uses ERP 600 PSM Former Budgeting. I need to offer an improvement of their system like migrationg to BCS but also some restructuring of master data and organizations.

So my question is, is it possible to reassign a newly created FM area to all company codes (at the moment they use 4 FM areas) even if there are postings in the old ones?

I've tried to test that but an error message appears: Message no. FI636 "You must not change assignment to company code &".

Is there some tool or enhancement for that? Or is a new system the only solution?

I've been searching through notes but haven't come upon anything.

Thanks in advance,

Toni