change roleManager - defaultProvider at RunTime / programmatically ??

i like to change the default rolemanager provider  at runtime  .. is this possible ? 

here is some code from ASP.NET - web.config :

    <roleManager enabled="true" defaultProvider="SqlRoleManager_Cars">
      <providers>
        <clear />
        <add connectionStringName="Conn_String2"
                 applicationName="Assets"
                 name="SqlRoleManager_Assets"
                 type="System.Web.Security.SqlRoleProvider" />

        <add connectionStringName="Conn_String2"
                 applicationName="Cars"
                name="SqlRoleManager_Cars"
                type="System.Web.Security.SqlRoleProvider" />

      </providers>
    </roleManager>

i want to change defaultProvider="SqlRoleManager_Cars"  to   >>  "SqlRoleManager_Assets"  

how to programmatically select it  ??
i am using same DB on same hosted server ..  ASP.Net framework 3.5 .

any help would be appreciated ...

Thank you ..