Update, I can step through each table's fields collection, but I cannot modify the value. It appears to be readonly when the field class is a part of the tables collection:

FOR EACH oxt in oxa.Tables
   FOR EACH oxf in oxt.Fields
       lcFieldName = STRCONV(oxf.xmlname,10) && shows the fieldname
       lcFieldName = UPPER(lcFieldName) && Change it to uppercase
       *--the next line bombs - value is readonly
       oxf.xmlName = STRCONV(lcFieldName,5) && Store the uppercase value
  ENDFOR
ENDFOR