The help file format for both HV1 (VS 2010) and HV2 is basically the same. HV2 has a few more meta statements but these wont break backward compatibility.

Note that the help engine and data store for HV2 has changed. So executables (Help Manager / Installer / Viewer etc) and API calls have changed. If you make calls to the installer or viewer or directly interact with the help data store or help API then you need to make some changes.