Solution with snapshot for all fields unchecked works fine for me. When archiving, I add tag with disk number to photos, and at that time I leave everything unchecked except for one field. This leaves data in all other fields, except for checked one. With one snapshot called "empty" (with all but one fields unchecked) and one snapshot called "normal" (with all fields checked), I can easily switch between normal IPTC layout and this "empty" one.
But if you have all fields checked, then of course it will replace all fields.