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)