Emacs 'destroys' latex files

[Pseudo]

Hi,

maybe this is a stuid newbe question, but since some time my
GNU Emacs 21.4.1 (on a debian machine) behaves in a way that it
displays greek letters or symbols as they do appear in a printed form.
I admit, that this looks quite pretty, but when saving one encounters a

big problem  if there is anywhere a \renewcommand appearing. To explain

the problem, consider e.g. a setting

\renewcommand {\AA} {\mathbb A}.

When opening the file Emacs will replace *all* \AA in the file by a
character
that looks like an A with little o on its head (the standard latex
meaning of \AA).
When saving, this behaviour destroys entirely the intention of my
renewcommand!
This behaviour is so annoying that in the moment I prefer running emacs
in a console mode even if I thereby loose some other nice functionality
..

Question: is there a way to stop emacs from doing so? If possible
please
don't explain me that \renewcommand is stupid anyhow ...

thank you very much!   Pseudo