Thank you for posting your question. I'm more than happy to assist you today. At any time you can request a Primary Domain Change if you want to use a different domain as your primary domain on your account. Here's a link explaining how to change your primary domain:
How to change your Primary Domain
As for the differences, add-on domains are separate, additional domains on your account. The main difference is that, add-on domain's files are stored in a folder inside your primary domain's root (public_html folder).
If you need further assistance please feel free to contact us.
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