Hi,
A couple of years ago, we made it so that when users sign up, they would have to use an x10Hosting subdomain as their "main" domain, and they can subsequently add different domains as "parked" domains. We put this in place because when users changed their "main" domain, it would cause lots of problems, whereas parked domains did not. Parked domains are no different to main domains really - they display the same thing as your main domain, and treated the same way to your main domain in cPanel. Re-signing up to our hosting service will not change this, as you will be asked to use an x10Hosting subdomain for your main domain and add parked domains later.
Could you please be more specific on what issues you are having when adding the domain as a parked domain? I'm sure we can get it sorted out, as parked domains are literally a mirror of your main domain.
For the subdomain problem you mentioned, cPanel should also show your parked domain in the dropdown. This means that you can add subdomains for both main domains and parked domains.
I'm not entirely sure what you mean by "still depending on the old domain", but if you are referring to the software on your site, this is normally something you can change pretty easily in the configuration.
Thank you,