When you do an update of people using import, and choose "Supporter/Non-supporter" or "Prospect", this will override the current value for that person, even if you didn't tick "overwrite".
This is in direct contradiction with the info provided at http://nationbuilder.com/mshelden/supporter_nonsupporter_prospect
and creates a problem, as people may accidentally turn prospects into non-supporters/supporters without realising.
We have spent some time thinking this through and have received other reports of this behavior being troublesome for customers. We have made a change so that this status will only change for existing people in your import if you choose to overwrite data.
Thank you for your help uncovering erroneous documentation! Taylor and our Services Team confirmed that support status is separate from "overwrite existing data," and must be indicated each time an import happens.
The customer question asking about how support status relates to imports updating existing records had an incorrect response. I have updated it and attempted to explain why we expect an import to update prospects to supporters and non-supporters.
If you want to update prospect profiles via a one-time import and keep them as prospects, you'll need to separate the import and select "prospects" as the support status.
I also updated the support status HOWTO to add this information.