Hi there, 

Good job on converting sub-customers to projects. At this time, there isn't an option to convert a project to sub-customers. I know how useful this feature is to you and I recommend sending feedback to our engineers through the gear icon. To do this, click the gear icon and look for Feedback. New product ideas and improvements are largely based on the feedback/suggestions we get from users like you. 

Feel free to ask any questions. I'm here to help. For future reference, here are some helpful articles to take a look into on your spare time:

 Projects FAQs

Convert existing sub-customers into project