There were some notes from Mark on the Houdini list about rendering to the old format
"You can also save out directly to:
Houdini 11 also reads/writes this extension. For example:
% gconvert foo.geo foo.bhclassic
The environment variable is more pervasive, so that .hip files will be
saved using the older .bgeo/.geo format.
Please note that the older .bgeo format doesn't support all features
of the new geometry library. It would likely be better to not use the
environment variable and just use .bhclassic when needed. Just my
opinion of course."