I'm running MEP 2013 and when i send out my electrical conduit files to people who are using 2012 and lower, all the conduits come in square.
I've tried saving the files down to 2004 and it still does nothing for users that are using 2012 or lower.
Good luck with that! We are still using 2012 until all our extended team decides together to move to 2013, mainly because of the compatibility issue you present. The objects in 2013 are 2013, and there is no way to save the "object" to a lower version. There is a way to see a "dumby" version of 3D objects without the squares, if that works for you. Put your 2013 model into Isometric view, then Export to the 2010 format. The objects will be 3D round but not smart.
You can also use the CONVERTTO3DSOLID command to convert all of your objects to 3D solids and save down to a version prior to 2013.