If I change the setting of "Journey to the Centre of the Earth" without changing the characters, how would the story be different and why is it so? If...

It
depends on the type of setting you change it to. The novel is a tale of adventure (the science
of this work is completely off-base, as opposed to some of Verne's other works, most notable
20,000 Leagues Under the Sea), and it is pretty standard. You have the
over-confident leader, with a naive/nervous follower, and a faithful companion. Put them in any
dangerous setting, and the characters at least would stay the same.

In a
voyage through an uncharted jungle (toward a rumored lost civilization), Professor Liedenbrock
would still be the "expert" on this civilization....