A family of Microsoft word processing software products for creating web, email, and print documents.
You can't make LISTNUM fields generate caps, but you can change them to AUTONUM fields and add the \* ALPHABETIC switch (where the switch is in caps).
{ AUTONUM \* ALPHABETIC }
However, the AUTONUM field does not support the switches for list level or starting value that the LISTNUM field has.
If you need to get rid of the period or other separator that's always included in the AUTONUM field's result, instead use an AUTONUMLGL field with the \e switch (in the Insert > Field dialog, that switch is added by the check box "Display number in legal format without a trailing period").