This question seems vague to me.
Something seems to have been left out.
To correctly answer the question *I believe* was asked would require more
detail on your part.

Obviously you have ordered or are considering ordering the Genuine Advantage
Kit...
http://www.microsoft.com/genuine/downloads/FAQ.aspx
----------
Q:What are the details of the genuine Windows offer?
A:To help customers who unknowingly purchased a counterfeit version of
Windows XP, Microsoft has created two genuine Windows offers for those who
qualify:

- Complimentary offer: Microsoft will make a complimentary copy of Windows
XP available to customers who have been sold counterfeit Windows. Customers
will be required to submit a proof of purchase, the counterfeit CD, and a
counterfeit report with details of their purchase. Only high-quality
counterfeit Windows will qualify for the complimentary offer.

- Electronic License Key Offer: Microsoft will offer an alternative for
customers who find out via the WGA validation process that they are not
running genuine Windows, but do not qualify for, or choose not to take
advantage of, the complimentary offer. These customers will be able to
license a Windows Genuine Advantage Kit for Windows XP online for a price of
$99 for Windows XP Home edition or $149 for Windows XP Professional. The
Windows Genuine Advantage Kit for Windows XP will include a new 25-character
Product Key and a Windows Product Key Update tool that will allow customers
to convert their counterfeit copy to genuine Windows XP electronically.
----------

You would only do this if you believe your copy of Windows XP (Professional
I assume from your subject).  Technically you are purchasing a valid 25
character product key for Windows XP - and not at much savings over buying
one new (unless you qualified for the complimentary offer.)

What do you *mean* by upgrading?
- Will the computer case and some internal components be the same?
- Will there be two or more computers in existance after the "upgrade"
where before there was only one?

My address to all MVPs and Microsoft® developers’ network formally known as
MSDN, I am a proficient Microsoft® Windows® user. I am able to do any job
task in Microsoft® Windows® software, as the current issue is concern it is
the issue of my expertise. I  wanna to tell everyone of you all that I have
the expertise to make copy of any Microsoft® software like Windows® and
Office® as legal and Genuine Validate because I have that technology
developed by Microsoft® corporation, I am a half software developer. I am
able to activate and Genuine Validate any copy of MS Windows® like Windows®
XP™, Windows® Vista™, and Office. I know The technological measures used in
Genuine Windows® Activation and Genuine Validation like Windows Product
Activation WPA, Microsoft Product Activation MPA mechanism and how its work.
It is the technology that I used  has developed by Microsoft itself I can’t
make that technology but implement that technology, So now I am able to
completely and successfully do that with out any problem or condition (MPA,
WPA, WGA)