The only thing that can happen is if Google indexes an http and https version of the same page. It isn't a HUGE deal... just depends on how obsessed you are about the site structure.
You could potentially have visitors start linking to the https version vs the http version, in which case that would be a problem.
Depending on the complexity of your site and code, this rule may be able to help you... where it says, pattern="download" that means that if someone visits the downloads section of your website, it will allow for https. You can continue to add additional folders by just repeating that code and replacing "download" with whatever the folder names are that you want to allow to use HTTPS. I did a Google search of site:lucid8.com inurl:https and noticed that your download section was indexed as using HTTPS, that is why I used it in this example.
If you are just using simple folder structures, this rule is not too bad to implement. I previously just implemented it with pattern matching and that was not fun.
Any way, if it helps great, if not, just try to keep your internal linking as consistent as possible. Sometimes the best way to do this is use absolute paths vs relative.
Mike
<rule name="AllHTTPexceptSIGNIN" stopprocessing="true"><conditions><add input="{HTTP_HOST}" pattern="<a href=" http:="" (www.)(.)$"="">http://(www.)(.)$" negate="true" />
<add input="{HTTPS}" pattern="on"><add input="{URL}" pattern="download" negate="true"></add></add></add></conditions>
<action type="Redirect" url="<a href=" http:="" {http_host}="" {r:0"="">http://{HTTP_HOST}/{R:0}" redirectType="Permanent"/>
</action></rule>