Yes @Julien_RENAUD you’re 100% right regarding the Guest policy! 
As for converting an Organization into a Workspace, I agree with you too; this is not something we usually recommend, as Organizations offer way more options and flexibility than Workspaces.
I’ve shared the steps to convert an Org back into a Workspace in the following post if you want to check it out: Help! Need to Convert Organization Back to Workspace! - #3 by Marie
As always, let me know if you have follow-up questions!