What is the difference between new and old type library (TLB) format?

Question

I just cannot find any explanation about the difference between new TLB file format and old TLB file format. This option is available for Visual Studio C++ projects, under Midl compiler options.

In the project file (.vcxproj), the option looks like this:

<Midl>
  <TypeLibFormat>OldFormat</TypeLibFormat>
</Midl>

Does anyone know the difference between new TLB file format and old TLB file format, and any possible collateral effects of changing to the new file format (e.g. COM issues, old VB6 projects requiring old format, etc)?