Before we can call the screen with the characteristic value to change we need to convert data from our reference data structure to table of type TT_API_VALI. Following method shows how to do it.
 
Importing:

I_ATNAMTYPE API_VALI-ATNAM ->Characteristic Name
I_DATATYPE REF TO DATA-> our reference data

Changing:

CT_VALUES TYPE TT_API_VALI-> Structure for characteristic screen

Exceptions:

NO_DATA_TO_CHANGE

Implementation:

 method move_ref_data_to_screen_val.
    field-symbols<ms> type any,
                   <any> type any,
                   <any2> type any,
                   <table> type any table,
                   <line> type any,
                   <p> type any,
                   <val> type api_vali.
    datam_p(16type p.
    datamo_type type ref to cl_abap_elemdescr.
    datam_format type cabn-atfor.
    datam_currency type rctmv-waers.
    datam_atinn type api_vali-atinn.

    if i_data is initial.
      raise no_data_to_change.
    endif.


    "get characteristic ID
    call function 'COC1_FEATURE_CHECK'
      exporting
*       feature_id                  = atinn
        feature_neutral_name        i_atnam
      
importing
        format                      m_format
        feature_id                  
m_atinn
*       feature_neutral_name        = atnam
*       conversion_exit             = l_atkon
*       number_of_positions         = l_anzst
*       set_get_parameter           = l_atpid
      exceptions
        invalid_class_type          1
        missing_feature_information 2
        no_feature_found            3
        no_feature_valid            4
        no_language                 5
        others                      6.

    if sy-subrc <> 0.
*   Error
*    raise characteristic_unknown.
    endif.

    refresh ct_values[].
    assign i_data->to <ms>.
    if sy-subrc eq 0.

      assign component i_atnam of structure <ms> to <table>.
      if sy-subrc eq 0.

        loop at <table> assigning <line>.
          insert initial line into table ct_values assigning <val>.
          if sy-subrc eq 0.
            <val>-atnam i_atnam.
            <val>-atinn m_atinn.
            <val>-atzhl '000'.
            case m_format.
              when 'NUM' or 'DATE' or 'TIME'.
                assign component 'LOW' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atflv = zcl_abapblog_com_classific=>convert_char_value(
                                                   i_atnam       i_atnam
                                                   i_value_new   
<any>
                                                      
).
                endif.
                assign component 'HIGH' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atflb zcl_abapblog_com_classific=>convert_char_value(
                                                    i_atnam       i_atnam
                                                    i_value_new   
<any>
                                                    
).
                endif.
                assign component 'UNIT_FROM' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atawe <any>.
                endif.
                assign component 'UNIT_TO' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-ataw1  <any>.
                endif.
              when 'CURR'.

                assign component 'LOW' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atflv zcl_abapblog_com_classific=>convert_char_value(
                                                                        i_atnam       i_atnam
                                                                        i_value_new   
<any>
                                                                        
).

                endif.
                assign component 'HIGH' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atflb zcl_abapblog_com_classific=>convert_char_value(
                                                                             i_atnam       i_atnam
                                                                             i_value_new   
<any>
                                                                             
).

                endif.
                assign component 'CURRENCY_FROM' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atawe <any>.
                endif.
                assign component 'CURRENCY_TO' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-ataw1 <any>.
                endif.
              when 'CHAR'.
                assign component 'LOW' of structure <line> to <any>.
                if sy-subrc eq 0.
                  <val>-atwrt <any>.
                endif.

            endcase.

            unassign: <any><any2>.
            assign component 'OPTLOW' of structure <line> to <any>.
            if sy-subrc eq 0.
              case <any>.
                when 'EQ'.
                  <val>-atcod 1.
                when 'GE'.
                  assign component 'OPTHIGH' of structure <line> to <any2>.
                  if sy-subrc eq 0.
                    case <any2>.
                      when 'LT'.
                        <val>-atcod 2.
                      when 'LE'.
                        <val>-atcod 3.
                      when space.
                        <val>-atcod 9.
                    endcase.
                  else.
                    <val>-atcod 9.
                  endif.
                when 'GT'.
                  assign component 'OPTHIGH' of structure <line> to <any2>.
                  if sy-subrc eq 0.
                    case <any2>.
                      when 'LT'.
                        <val>-atcod 4.
                      when 'LE'.
                        <val>-atcod 5.
                      when space.
                        <val>-atcod 8.
                    endcase.
                  else.
                    <val>-atcod 8.
                  endif.
                when 'LT'.
                  <val>-atcod 6.
                when 'LE'.
                  <val>-atcod 7.
              endcase.
            endif.
          endif.
        endloop.
      endif.
    endif.
  endmethod.                    "move_ref_data_to_screen_val