In your main assembly, find your sub assembly, and just change it's LOD.
You can also link LODs with productivity tools, so that you can manipulate more sub assemblies LODs in main assembly.
What Greg said is true, but also understand it will want to create an LOD in the upper level assembly as well.
Access a broad range of knowledge to help get the most out of your products and services.
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.
Upgrading to a 2015 product? Make sure to check these out 1st!