I'm not clear why this requirement exists in the import function. When you update a list, sometimes the people are a mixture of the two and to have to select one or the other is detrimental to the database. What is the purpose?
Perhaps an update function on the import could be done.
It also brings up that one can't select the supporter radio button but rather has to select either strong or weak support. Once again, not sure why that is the case.
Updated response 04/20/17:
We understand that sometimes your import can have a mixture of existing people and new people. We've made a change that ensures that people that already exist in the database will not have their status changed by an import unless you have the "overwrite" box checked.
Updated response 10/13/16:
I'm going to try to completely answer your questions and include information regarding the technical under-pinnings of NationBuilder. Within the code, support status is a binary state - a profile is either a supporter or a non-supporter. Prospect is a secondary flag, allowing a nation to send email blasts to someone who has not opted into communication. A nation is designed to move people to action -- action that helps the nation achieve its goals. Therefore, one of the first goals of a nation should be to encourage prospects to opt into communication.
Gradations of whether someone is a supporter or non-supporter can be defined using support level -- A "one" is the strongest level of support, a "five" is actively working against the nation. Within this five-point scale, "three" is undecided, a state sometimes referred to as "prospect."
Most email marketing software does not allow customers to send email to people who have not opted into communication. Emailing prospects is a difficult thing to do well and the strategy for emailing prospects should always be different from supporter mailings.
So, NationBuilder is built around the expectation that in your initial communication with prospects, your primary goal is to sort them between supporters and non-supporters. Non-supporters can be turned into supporters, but really after the first one or two emails to a prospect, you should stop emailing them.
Therefore, if your import updates profiles that already exist in your nation, we expect that any prospect you contacted has now been defined as either a supporter or a non-supporter. If your import updates existing profiles and you select "supporter / non-supporter," and support level is imported, then you will have complete control over whether people are marked supporters or non-supporters. Imports that update profiles without support level will have support status re-determined based on the amount of information contained in the profiles.
If you truly want to update prospect profiles and retain the prospect status, you will need to create a separate import and indicate that all records within the import are prospects.
Functionally, you experience support status as one of three options. The way batch updating prospect status works hints at the fact that these three labels are handled differently. I admit that this is confusing -- it took me several months to dig into how the process works in order to write the support status HOWTO and to be honest, I did not consider updating profiles via import when I originally wrote it. (I will now update the HOWTO to reflect this.)
This functionality may seem like a bug, particularly since incorrect information was previously written in response to your question. I hope that this more complete explanation of how NationBuilder works helps clarify things.