Migrated In domain & PN Servers

[#1]

Now I'm really confused !
I've migrated in a new domain (www.mynewdomain.co.uk)and it's showing my trial website on cgi.myusername.plus.com (which is fine!). I thought I would have had to move everything over from the latter to www.mynewdomain.co.uk !
Can anyone explain the differences between the various servers and domains on PN, and the significance of them ?
I have webspace, (presumably on one server ?) under www.username.plus.com, I then have a separate server space (?) on cgi.username.plus.com, but this appears to use the same webspace (?)
I already store php scripts on something (?) called RUMPUS, which must be linked to the cgi.space, and now FORCE9 has come into the equation, (I had to change the migration IPSTAG to FORCE9, but am I actually on that ?)along with the new domain name (www.mynewdomain.co.uk), and all using the same webspace.
I presumably am not comprehending the difference between these various entities, or the significance of the differences, and/or the relationship between them.
Can anyone comprehensively explain it and/or point me to a suitable explanation ?
As my old cgi space has already been indexed by google, how can I divert all to the new domain, when the pages are one and the same?
I understand how to use "diverts" and "No Index" on obsolete pages, but this wouldn't seen to be applicable when they are the same pages ?. 
Any advice appreciated.
Many Thanks

[#2]

I could be wrong on some parts but I'll give it a whirl.
Rumpus (and Humbug) are the MySQL servers which you need to connect to (not localhost) with scripts that require database access.
ccgi.username.plus.com is your super webspace letting you use PHP/CGI etc, scripting technologies, whilst username.plus.com is bog standard web space where you can only use HTML files, PHP files will not parse on that space as it doesn't support the features.
You can have your domain name point at username.plus.com or ccgi.username.plus.com, google will translate that into your domain name (you can actually use both to get you there, yourdomainname or username.plus.com) so as far as google is concerned it shouldn't notice a difference, in theory.
If memory serves me correctly, your cgi webspace is very small (50mb?) while your normal webspace is 250Mb, and that is also your daily limit for transfer.

[#3]

Thank you David,
That's helped put things in perspective.
My new site is only using around 1MB at present so I shouldn't have anything to worry about for a while ?
Although it sounds as if it's OK to leave things as they are at present, I may at some stage want to put up some completely separate webpages on the old cgi.username.plus.com domain.
That being the case, should I look to disassociate it from my www.newusername.co.uk domain, whilst still retaining any search traffic that the old site may have generated ? And if so, what method should I look to ?
Regards

[#4]

Difficult one that, if you don't require PHP/MySQL/CGI then you can use the normal webspace and have the domain pointed there, but if it's essential you could make a subdirectory, so ccgi.username.plus.com/subdirectory and send people there whilst leaving the root directory alone.  I'm not sure if PlusNet offer subdomains, so you could have subdomain.domainname.com point to ccgi.username.plus.com/subdirectory and www.domainname.com pointing to the root, you may be able to as PlusNet offer complete(?) DNS control so you should be able to set it up like that, but I'm no expert on DNS so wouldn't want to break your website!