Something you have to take into consideration is the fact that someone else brought up earlier in the thread

simply installing a panel on top of an existing structure is going to cause problems in the long run. Each panel has it's own way of doing things (installing apache, for example) weather it be in a different directory, or with different options.

Not to mention, with apache, when you install the new panel, you will lose all your vHosts, and everything in favor of the new panel. The panel essentally takes over ALL of the management of those services, and will only do it ITS way

If you are serious about installing a panel, you need to BACK UP ALL YOUR SITES before you even come close to installing it. Then it's best to start with a clean VPS (reload the OS)

You can use an open source control panel if you prefer. There's no need. A few good ones are ISPConfig, Kloxo, and my favorite, VirtualMin GPL

Kloxo is good for free

sir if u dont control panel u can try webmin its free also there is other free and if u dont wanna use control panel u can do what ever u want from ssh

What do you think about ispmanager?

You can use Webmin, web-cp, kloxo and any other free control panel.

Yes he can install a control panel without any restriction and he can even install free ones such as Webmin which works great in most cases.

there shouldnt be any restrictions and webmin should do the trick.

Yes you can Install any control panel at anytime. But better to talk your hosting provider bcz they have low cost control panels.

I would do a manual analog back up first in case the other back up fails

Yes he can purchase cPanel license from other provider and get help for its installation for example but this will not be the most lucrative offer for sure.