What is the purpose of any kind of upgrade........

Well yes and no...often an upgrade or "signature" model is designed around one of the manufacturers best selling items...and the upgrade is often "trickled" down from a higher model...such as the Vandersteen 2ce signature which features a tweeter from model 3...among other things...but to answer your excellent question...is the additional cost worth it yet? I have mixed views...I will use Vandersteen as an example...you could also use Meadowlark Hotrod...or a number of products...for these...the upgrade models cost roughly 4-500$ more(APPROXIMATLEY 1/3 OF THE ORIGINAL PRICE)...and since they are applied already to a speaker in production...the additional cost for the manufacturer is not that much greater...the kicker: are u really making a significant gain in performance to justify the extra...in absolute terms...the gain is marginal...we are afterall talking about VERY GOOD SPEAKERS to begin with...however...if u see yourself owning these speakers for a considerable amount of time...and want the most current design...then the upgrade is worth investigating...although you will be severly testing the law of diminishing returns

Mamba, I've upgraded my turntable through the years as the improvements bacame available. I'm glad I did. I've owned the piece for almost 20 years and it performs equally with what is available new today. Upgradeability is a strong reason to buy an item in my book.
Patrick

Why should I buy such a product anyway and pay again for something that was already paid?

Don't think that you are paying twice. The point is to spread the component out to more price points, knowing that audiophiles tend to upgrade. I think it is easier than going through the buy-sell-buy hassle, if you happen to really like the component. The manufacturer benefits from keeping an intro buyer buying from them.