Source Referrer Won't Change

We use a form's hidden field to capture the referrer value (ref=xxx) in the URL's query string. Lead Source: hidden URL parameter: ref.

This value populates the "Source" field in the campaign's Smart List. It worked well so we used the same concept in another form.

Problem: if you had completed the first form and now, maybe a month or two later, click-through to the new form (with new query string) and complete the new form, the Smart List will be populated with your original referrer (source) value -- and not the value in the query string of the URL that delivered you to the new form.

I tried (new form) changing the query string to src=xxx, and Lead Source: hidden URL parameter: src - didn't work.

Thanks for any advice.