Update BSEG-ZUONR with Function Module - abap

Question

I successfully change BKPF-BKTXT with FM CHANGE_DOCUMENT but why can't I change BSEG-ZUONR with FM CHANGE_DOCUMENT too?
Here's the FM CHANGE_DOCUMENT:
CALL FUNCTION 'CHANGE_DOCUMENT'
TABLES
T_BKDF = t_bkdf
T_BKPF = t_bkpf
T_BSEC = t_bsec
T_BSED = t_bsed
T_BSEG = t_bseg
T_BSET = t_bset
* T_BSEG_ADD =
.
Here's the code to change BKPF-BKTXT (succeeded):
wa_t_bkpf-mandt = sy-mandt.
wa_t_bkpf-bukrs = '1000'.
wa_t_bkpf-gjahr = gjahr_import.
wa_t_bkpf-belnr = belnr_import.
wa_t_bkpf-bktxt = zuonr_import.
APPEND wa_t_bkpf TO t_bkpf.
Here's the code to change BSEG-ZUONR (failed):
wa_t_bseg-mandt = sy-mandt.
wa_t_bseg-bukrs = '1000'.
wa_t_bseg-gjahr = gjahr_import.
wa_t_bseg-belnr = belnr_import.
wa_t_bseg-buzei = '1'.
wa_t_bseg-zuonr = zuonr_import.
APPEND wa_t_bseg TO t_bseg.

Answer 1

As author has no time to confirm, I can do this for him as I just tested this case.
If we pass to FM all parameters from its signature the update runs smoothly. For example, like this:
DATA: lt_bkdf TYPE TABLE OF bkdf,
lt_bkpf TYPE TABLE OF bkpf,
wa_bkpf TYPE bkpf,
lt_bsec TYPE TABLE OF bsec,
wa_bseg TYPE bseg,
lt_bsed TYPE TABLE OF bsed,
lt_bseg TYPE TABLE OF bseg,
lt_bset TYPE TABLE OF bset.
wa_bkpf-mandt = sy-mandt.
wa_bkpf-bukrs = '5900'.
wa_bkpf-gjahr = gjahr_import.
wa_bkpf-belnr = belnr_import.
wa_bkpf-bktxt = 'Batch'.
APPEND wa_bkpf TO lt_bkpf.
wa_bseg-mandt = sy-mandt.
wa_bseg-bukrs = '5900'.
wa_bseg-gjahr = gjahr_import.
wa_bseg-belnr = belnr_import.
wa_bseg-buzei = '1'.
wa_bseg-zuonr = '20151131'.
APPEND wa_bseg TO lt_bseg.
CALL FUNCTION 'CHANGE_DOCUMENT'
TABLES
t_bkdf = lt_bkdf
t_bkpf = lt_bkpf
t_bsec = lt_bsec
t_bsed = lt_bsed
t_bseg = lt_bseg
t_bset = lt_bset
.
COMMIT WORK.
All FM table parameters except the last one are mandatory.

Answer 2

Do not use this FM
CALL FUNCTION 'CHANGE_DOCUMENT'
This FM is changing all other fields to initial if not provided.
CALL FUNCTION 'FI_DOCUMENT_CHANGE'
It seems that this FM cannot be used to change line item which has account type (BSEG-KOART) - 'S' (GL Account).
Try this FM:
'FI_ITEMS_MASS_CHANGE'

Answer 3

The field zuonr references to an object it belongs to.
For example a purchase order.
Lets asume you pay a position of a purchase order.
A document in bkpf/bseg is created (and more).
Bseg-Zuonr contains the number of this purchase order position.
If you were allowed to change this field, you would destroy the referential integrity of the data. It would point to a purchase order position it was not created from or one that doesn't exist at all.
So from a business standpoint it makes no sense to ever change this field after it is created, therefore SAP will never allow to change it.