[XeTeX] TeXLive update issues

[Gareth Hughes]

George N. White III wrote:
> What you need is some assurance from the TL maintainers that the
> CTAN version of TL is and will remain self-contained: e.g., everything
> is contained under one top level directory and the only external change
> needed is to put the "bin" directory at the start of the PATH.  Note that
> when a distro packages TL they tend to put stuff in other locations.
> If you rely on additional evironment settings or symbolic links into
> standard "bin" directories then you loose the self-contained property.
> 
> If you use a command-line, it is not a big deal to adjust the PATH to switch
> between different TL versions, one of which can be the one provided
> by Ubuntu.  Many people do this routinely, e.g, to have a current TeX
> for some projects while maintaining the ability to test things with the
> version from a linux distro.
> 
> With a pure GUI approach things may not be so easy, but some
> tools provide a way to configure the PATH used for the TeX programs,
> and it is possible to wrap a GUI program in a script that adjusts the
> PATH.  Finally, if all else fails there are virtual machines that allow
> you to have a complete system for each TeX version.

I am glad to announce that I now have fully functioning version of
TeXLive 2009! It seemed that, as Karl pointed out, setting TEXMFCNF is a
bad idea. After unsetting, updating TeX and rebooting, all is working
well, and kpsewhich and texdoc are finding their relevant files. I've
still got the repository version of 2007, but it is dormant. Which makes
me wonder:

1. In the unlikely event of the repository updating any of the TeX
packages, would the update be likely to scupper my 2009 settings? Or is
there just one package I should be wary of?
2. I understand that TeX looks for files first on the home tree, then
the local tree, then the main tree. So, you are expected to have older
versions in the higher trees. Is there a way that I can get TeX
ultimately to search my 2007 distro if it draws a blank in 2009?
3. What would be the best way to set up a command-line method of quickly
processing via my 2007 distro rather than through the now default 2009?

I ask this because my old XeLaTeX files make extensive use of the old
version of bidi.sty, and many need to be revised in order to work with
the newer version.

Gareth.