Check out this announcement email about the syntax change:
https://lists.gnu.org/archive/html/emacs-orgmode/2015-12/msg00525.html

It has a snippet to convert the old syntax files to new syntax.

--
Kaushal Modi
On Feb 9, 2016 6:45 PM, "Myles English" <mylesenglish@gmail.com> wrote:

> Hello Julien,
>
> Julien Cubizolles writes:
>
> > The #+BEGIN_LaTeX... #+END_LaTeX has been deprecated in favor of
> > #+BEGIN_EXPORT latex ... #+END_EXPORT. I have however a lot of older org
> > files that I will need to export again. I can manually change the
> > blocks but I was wondering if some automatic conversion function has
> > been implemented.
>
> I don't know if there is such a function but, for the problem you
> describe, I would try something like this:
>
> sed -i 's/#+\(BEGIN\|END\)_LaTeX/#+\1_EXPORT/' myfile.org
>
> If there are lots of files maybe (UNTESTED):
>
> grep -rl "BEGIN_LaTeX\|END_LaTeX" | \
>      xargs -IXX \
>            sed -i's/#+\(BEGIN\|END\)_LaTeX/#+\1_EXPORT/' XX
>
> Myles
>
>