How do I upgrade my existing LyX system to version 1.3.0?
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One of the perennial bug bears of LyX users in the past has been that
they have had to run Edit->Reconfigure when starting their new version
of the code for the first time. Strange and wonderful things would
often result if they forgot to do this, so LyX 1.3.0 now runs
Edit->Reconfigure automatically the first time the program is run.

If you have your own layout files, you may need to update them a little:

- floats are now defined in the layout file, using the "Float"..."End"
  construct. In most cases, adding "Input stdfloats.inc" to your layout
  file is enough.

- counters are also defined in the layout files, using the
  "Counter"..."End" construct. As for floats, adding "Input
  stdfloats.inc" is probably a good idea.

And of course, if you upgrade from LyX 1.1.x, remember that since
1.2.0, you have to do the following changes:

- all layout files should have a "DefaultStyle" entry

- the "Latex" font style does not exist anymore. If you really need
  its functionality, consider using the "PassThru" keyword instead.

The new layout format keywords are described in the Customization
manual.

If you have your own binding files (especially math.bind), you will
have to update them

- math-insert now takes a latex macro name as argument, so that
  "math-insert sqrt" should now be "\math-insert \sqrt"

- math-greek-toggle is now gone, and should be replaced by explicit
  bindings like

  \bind "M-m g a"      "math-insert \alpha"


Build requirements
------------------

LyX's graphics handling system has changed substantially. If you
do not have the JPEG library installed, you may need to install
it before you can use the graphics capabilities of LyX. If you
do not have the ImageMagick command-line tools installed, you
will need to modify the default set up of LyX, or install them,
in order to get previews of your document's graphics.

Document transfer
-----------------

LyX 1.3.0 uses an external python script, lyx2lyx, to import documents
written using previous versions of LyX. All versions of LyX as far back as
0.12 are supported, so any klyx users still holding out for an alternative
to xforms will finally be able to put their dinosaur to rest ;-)

Of course, this means that you must have python (at least version 1.5)
installed in order to use LyX 1.3.0 with your old documents.

lyx2lyx also has the framework in place to be able to convert documents
to an earlier format. However, these converters have not yet been written
so older versions of LyX will NOT be able to read documents saved with
LyX 1.3.0.

If you were previously (in LyX 1.1.x) using the floatflt paragraph
option to wrap text around a figure, it was necessary to modify this
for LyX 1.2.0 manually, as described in the manuals. The feature has
been re-implemented as "Floting figure" inset in 1.3.0. Old files will
be converted automatically, but you may want to convert the
1.2.x-style ERT constructs with the native solution (see section 3.8
of the Extended Features manual).

Since LyX 1.2.0, the babel package is loaded after the user-defined
preamble (because some packages really need to be loaded before
babel). If you relied, on babel being loaded before your own
definitions, you can add an extra "\usepackage{babel}" statement at
the beginning of your preamble.

http://bugzilla.lyx.org/show_bug.cgi?id=315