Is it acceptable to take an existing patent, copy and paste the
description and claims, and then change the references to the
technology that is driving the invention?

Example: I invent a tennis racket that uses a magnetic force field
instead of strings. Can I take the original tennis racket patent,
replace any references to string/wire with ?magnetic force field?, and
file this as a new invention?

The problem is not in the copying of the previous application (i.e.
copyright).  The problem is that the specification will not be
enabling for your invention.  A specification discribing a string
racket will not enable a force field racket.  Plus the claims will not
have support in the specification or descrive your invention.

It is better to write an application; however rough, that describes
your invention than to copy a well-written application that is not
enabling.  The first will get your a patent (if it is patentable), the
second will get an immediate rejection.

Maybe the string-magnetic force field comparison is too extreme; let's
use a different example. In this scenario, the tennis racket is made
out of wood.  Plastic materials exist, but nobody has ever thought
about making a plastic racket. Is it acceptable to take current
application and subsitute "wood" with "plastic" in an effort not to
recreate the wheel (ie the writeup of the claims, the product
description, possible applications, etc)?