Buying used vs new speakers from a technology perspective

Depends on how far back you go.  Example: speaker surrounds before the early-mid nineties were made from perishable materials.  Not anymore.  However, electronics always age but in my experience fail only occasionally.  A ten year old speaker should be fine unless it was really abused - which is rare

How can someone upgrade an old speaker with new technology and then say see how good my old speaker sounds? 

My AR 9's from 1979 still sound GREAT what a find used 

There has probably been a bigger improvement in more affordable speakers over the decades as improved materials and techniques from higher end speakers trickles down. The Kef LS50 is a good case in point where the driver technology is much better now than it was than when the Uni-Q was introduced several decades ago. Also, as a simple fact, offshoring manufacturing has improved price/performance ratios. Note that I am making no comment on whether this is a good or bad thing, or any shade thereof.

As a final comment, when you look across various manufacturers, you sometime come across u turns and blind alleys where through a change of designer or strategy one generation of product may be worse than the one that preceded it.

A company like Wilson Audio (for example)  has avoided this by having a very coherent set of design principles and then continually refining what it does based on those principles. As a result, there has been quite audible improvement in the speakers across generations.

However, the older Wilsons are still great buys used as they were fundamentally right to start with. The same applies to the various BBC based designs, and the Quad ESL 57.

Not being one who frequented shops, shows or other audiophiles' homes, I continued to greatly enjoy my 3 decade old Von Schweikert VR4.5 silver, as I gradually upgraded other components in my system.  Recently  I gave them away and purchased a new pair of REVEL Salon 2.  Honestly, the new ones improve on the old in many ways, but I still could have been happy continuing to live with the old....I guess the point is what one wishes....good sound or the feeling that they own what they have felt to be the best "value" for them......some need SOTA, some need best bang for the buck, some need a certain "sound"...to each there own, there is no right answer, of course....