Hi
You can see the system behaviour here in ME21n
LMEPOF00 -> FORM mepo_header_fill_zterm
...
* first process the payment term
ch_zterm = dekko-zterm.
IF aktyp EQ hin OR
gs_lfm1_changed EQ mmpur_yes OR
fekkoadd-refbs NE ofekkoadd-refbs OR
fekkoadd-anfnr NE ofekkoadd-anfnr OR
fekkoadd-konnr NE ofekkoadd-konnr.
IF fekkoadd-refbs NE space AND fekkoadd-refbs EQ rekko-ebeln.
ch_zterm = rekko-zterm. >>>
The payment terms are taken form the LFM1 table in the first instance as outlined previously.
If you then reference a PO subsequently you will see here that the new information is held in
the rekko table and this is the value taken over per design.
The options you have are a small modification above, or in the calling include LMEPOF2I there are
many enhancement points you could use, or when the user presses the check TAB user exit
EXIT_SAPMM06E_012 is called and you have al the information to reset the old ZTERMS value from the internal OEKKO table.
Hope it helps you further
Kind regards
Brian