I recently upgraded to Houdini Indie. Unforuntately, I was not able to open my work that I originally created through Houdini Apprentice.
The file format supported by Houdini Indie is hiplc (lc stands for limited commercial). If you open a hipnc file (non-commercial) inside of Houdini Indie, it will just instead open it through Houdini Apprentice and you will not be allowed to use any of the Indie features.
Thankfully, there is a way to convert between hipnc and hiplc.
This sounds like it is only available for a limited time, but from links pointing it dating back to late 2014 it seems to be as old as 2 years old.
Either it is still supported by sidefx or the conversion between hipnc and hiplc is so trivial that their conversion tool still works years later.