How to change your plane of refernece after assembly is made? Is is possible?

Yes, SolidWorks automatically "fixes" your first component inserted..

  1. Step 1:

    After you insert your first part, right click and select float. Your component will now be completely free in the assembly.

  2. Step 2:

    Mate your part planes with the standard planes in your assembly, the way you want it facing.