Do you want to change the template if it is only a certain category or a certain template?
In either case you can override the routine OverrideTemplate in the pages that render topic and categories (found in web.config).
Code:
<add name="Category" url="c-{CategoryID}-{SEName}.aspx" virtualPath="~/showcategory.aspx" checkPhysicalUrlAccess="false" />
<add name="Topic" url="t-{Topic}.aspx" virtualPath="~/driver.aspx" checkPhysicalUrlAccess="false" />
In the OverrideTemplate routine you can use logic to detect which category or topic you are looking at and then change the template based on that.
Otherwise you can use a method like the one discussed here: http://forums.aspdotnetstorefront.co...ad.php?t=22665
ML9.3.1.1
SQL 2012 Express
VS 2010
Azure VM