I'm a little rusty in this area, but what you are saying sounds correct. If a respondent begins a survey with pass-in fields, leaves, and then comes back in and you change the value of the pass-in fields, previously answered questions are cleared out as the software assumes the respondent is coming back in to start over. It sounds like you are linking more than one Sawtooth survey together? Is there a reason for that? If there is, you might be better suited by linking to a third survey so you can change/update pass-in fields.