As mentioned in note 827413 there is a golden rule regarding the changeability of fields in FI documents:
When the field is used in Special Ledger field movement then it is not possible to change this field in FI using transaction FB02.
The reason is that Special Ledger is only updated when the document is created.
For other components like profit center accounting or special ledger a subsequent change in an FI document is only local.
Special Ledger does not get the information about the change. Thus, in order to avoid inconsistencies between SL and FI those fields are not changeable.
So if you have change in FI document and also want to reflect it to SL document, you can refer below method:
1). Reverse FI document, then re-post the FI document
Or 2). Delete the SL document by program RGUREP03, then re-transfer the FI document to SL by t-cd: GCU1.