This is a complicated issue. The reason the Reform and Conservative have
changed is because Israel uses Sephard pronounciation. I was told this was
originally in respect for Ashkenaz Jews who did not like Hebrew being used
for profane use. Sephardim did not have the same problem. According to my
Hebrew teacher (Reform Ashkenaz, if that matters), the more accurate part
was just an excuse for using the Sephard which was "not really Holy
Hebrew." Some communities, I hear, still only use Yiddish for secular
dealings and reserve Hebrew for religious purposes.
In any case, it is used in Israel, and thus has become very commonly taught
as the only way to pronounce Hebrew. From what I know, it is not really a
particular Sephard dialect anyway. My synagogue pronounces things quite
differently from modern Hebrew. We pronounce "Qadosh" with a gutteral, we
pronounce "Hatanu" with a slightly hard h sound, we prounounce "Atoh" with
an a as in cat sound for the long and o as in mop sound for the short. I
originally thought it was the Yemenite manner that gave rise to Israeli
Hebrew, but they also pronounce Hebrew with their own variations. It may be
a European Sephard or American Sephard pronounciation I don't know about.
Our pronounciation is, of course, much closer to "Sephard" pronounciations.
"La'asos es ha'shabbas l'dorosam bris olam" is quite different from
"la'asot et ha'shabbat l'dorotam brit olam"! Not to be disrespectful, but
the first time I heard someone pronounce it that way, I was shocked into
laughter. It really took me by surprise. I never realized that a sentence I
was so familar with could be SOOO different. I went to an Ashkenaz Kabbalat
Shabbat service and totally missed the Barchu because I was trying to
figure out where everyone was and what was going on. The differences in the
service did not help the fact I couldn't recognize the words without great
difficulty.
Everyone has had to change though. Our Hebrew school teaches the modern
Hebrew "Sephard" the same way the Reform and Conservatives do. It is no
longer a matter of personal preference, since a whole nation has to
communicate with each other and invariably has to have only one dialect.
Pretty soon, the only way you will tell customs apart will probably be how
one binds tefillin since siddurim are combining the services and
pronounciation is being standardized.
It is unfortunate that you were ridiculed, but speaking Ashkenaz will get
that response from many (if not most) people outside the Orthodox community.