Changing your main domain won't create any problem, but there is more involved with changing a main domain than there is just adding or removing addon domains. It's not a problem if you know what to expect.
Right now, your main domain is a helioho.st domain, and your custom domain is the addon. You can keep that if it works for you.
Later on, if you want to change your main domain to your custom domain, what would happen is:
- We would need to delete your custom domain from being an addon domain, which would delete the folder inside Plesk for it, and all its files. If you have a backup made, this isn't really a problem, but you would need to re-upload the files after the changes are made.
- We would make your custom domain the new main domain, which would delete all the existing files inside httpdocs that belong to the free helioho.st domain you have now. This isn't really a problem either if you made a backup of the files you had. If you even wanted the helioho.st domain put back on to your account as an addon domain, we could do that (but you would need to re-upload all files).
The only difference I've noticed between my main domain and my addon domains is that the main domain files go into httpdocs, and my custom domain files don't. They go into a folder that has the same name as my custom domain. I actually prefer it this way so I don't accidentally upload things to the wrong place (since I use my main domain mostly for testing), but you may prefer it the other way around.
I hope the above was helpful but please let us know if you have any questions.