When you redirect the https pages to http, any inbound link juice that may have boosted the https pages but not the http pages will be redirected to the http version, so this isn't something I'd worry about. After the redirect, the http equivalents should be ranking.
I should tell you that I feel a 301 redirect is the only way to dependably fix this situation. I'd recommend you canonicalize your domain at the same time.
You might also want to run a test to see if your http pages return when you disable Google's dupe filter. Run the search, and then, in your address window, append &filter=0, click on "Go," and see if your http pages show up. This won't be an exact test of the situation you should expect after the redirect, but it might be helpful nevertheless. Note that extra pages may be returned on sites ahead of you in the serps, so your pages will be shifted down when you do this.