This MathGroup discussion should answer your question, so I'll cite it here:
On Tue, 29 May 2012 05:47:52 -0400 (EDT), JCW wrote:
Please forgive my dragging up ancient history: I have been using
from version 2.2 through 7.0. I remember at least one (maybe two?) format
conversions that were necessary to update old notebooks to the newer
formats. I also remember at least one batch-notebook-conversion command.
Unfortunately I cannot remember the details:
1) Between which versions did these format changes occur? (I'm pretty
sure there was one from the vs. 2.2 x.ma and x.mb format to the newer
x.nb format. Was there another format change within x.nb versions?)
2) What was the name of the batch converter (or converters) mentioned
above? (I know I have it in my archives, but it's difficult to find
without a name!)
Thanks in advance for any help!
I'm always one for ancient history.
There was only the one switch from 2.2 .ma/.mb files to 3.0 .nb files.
Possibly, you might be remembering v6 when pre-v6 notebooks were
opened with an compatibility tool added to the top, but the tool had
nothing to do with the file format...it was merely an aid to updating
some of the code inside the notebook.
Also, .nb files are not wholly backward compatible. They'll always
open in older versions, but as new features get added to Mathematica,
inputs/outputs depending upon those features may not render or
function correctly in older versions. The most radical such change
would be in v6, when we switched from using PostScript for graphics
to more native constructs in the notebooks. Which means that pretty
much any v6 or later notebook is going to produce graphics which are
wholly unusable by v5 or earlier (but the notebooks do open, and
textual content comes through just fine).
The converter for .ma->.nb is
Developer`NotebookConvert. It's still
documented in the latest shipping Mathematica.
jfultz at wolfram.com
User Interface Group
Wolfram Research, Inc.