Some articles as i collect them are:

• Actually referencing and making diff. levels work together
  • same recommendation as the link above (w. 433 upvotes) – useLegacyV2RuntimeActivationPolicy
  • and again but with a little more if you scroll down (I see a pattern here)
  • a few good links exist with this article as to what useLegacyV2RuntimeActivationPolicy does
    • msdn article on  <startup useLegacyV2RuntimeActivationPolicy…>
    • why it was done
• Background on CLR (major release and side-by-side) vs. upgrade to a CLR (inplace upgrade)