From my very prelimnary research I found out that Vegetarianism was
prevlant among a group of people in India for at least hundreds of
years. Is this true?

At what approximate time in history did this happen? I have read
aricles stating that this might have been true hundreds of years ago.
SOme claim it has been true for thousands of years.

Why did this happen? There are religious reasons but suddenly changing
eating habits especially in ancient times would have been very
difficult.

I am looking for well researched answers that sites relevant sources.

I'm sorry I don't know much about this (except to confirm that many
Hindus are vegetarians); but why are you so sure that this was a case
of "suddenly changing eating habits"? The animals from which we're
descended - primates - are vegetarian, and vegetation is more readily
available than meat or fish. So possibly vegetarianism in India is
more a matter of not having converted to meat-eating, rather than the
opposite.

You might look here:
http://en.wikipedia.org/wiki/Vegetarianism
http://www.utilitarian.net/singer/by/1995----02.htm
http://www.flex.com/~jai/articles/hinmeat.html

Agreed. I was born a Hindu vegetarian. You may be surprised that less
than 10% of Hindus are vegetarians.

I do not want to offend anyone but my question is purely for academic
informational purposes.

ALso I read in some books that most of the Hindu Brahmins (or their
forefathers) ate meat. Somewhere along history some change occured.

Contrary to your argument most animals are not vegetarians they are herbivourous.
There is a subtle difference -- Most Indian vegetarians drink milk
from non-human sources eg. from cows, buffaloes.

I dont believe that forefathers of present day Hindu vegetarians never
ate meat. Most present day Hindus descended from the tribes in the
URALS (called Aryans) or native (Dravidians). Both were meat eating
cultures.