Hi, I just purchased a Houdini Indie License in order to sell models for my upcoming Kickstarter. All I need to do is export .obj files. Apprentice can already do this. Would it be fine to continue to use Apprentice for this purpose considering I have already purchased an Indie License?

Better contact support. They could do a one-time conversion of your files. Then you can work with Indie instead.

About the commercial use of the Apprentice I believe the answer would be no, according to this (14. Definitions): https://www.sidefx.com/legal/license-agreement/ [www.sidefx.com]
It appears to me that you would need to use your Indie version for that.

"Non-Commercial License" means a License that is any of the following License Types: Apprentice License, Education License and Evaluation License."