The Transition to LaTeX2e

Latex2e has been available since 1994, and is increasingly commonly the default at toher sites. It is much more robust, and documents written using LaTeX2e are considerably more portable than those written using LaTeX209.
LaTeX2e provides a compatibility mode, so that LaTeX documents should be processed successfully by LaTeXe, but will not necessarily produce exactly identical output. If you just want to ensure that your old files can still be processed by latex, you should check the output produced when you run latex on the file unchanged. There may be some problems which need fixing, depending on how devious the style files you used were. If you are lucky, your file will work unchanged.

If you are writing new LaTeX documents...

... or updating old documents, you should use the new LaTeX2e \documentclass form of headings ("native mode"). Converting to LaTeX2e is essentially a matter of changing the first few lines of preamble in a document, from:

     \documentstyle[<options>,<packages>]{<class>}

to something like:

     \documentclass[<global-options>]{<class>}
     \usepackage[<local-options>]{<package>}
     \usepackage[<local-options>]{<package>}
     ...

Here, <global-options> are those which apply to the whole document, <local-options> apply just to particular packages. One common change is to replace the LaTeX option `a4' with the new global option `a4paper'.

Apart from this, there may be changes needed to handle special characters:

Some symbols are not declared by default, add:
\usepackage{latexsym}
A few accents and other characters are accessed differently.

Because we had the NFSS installed here, the font changing commands in our version of LaTeX behaved differently to the LaTeX's used at other sites. (This is exactly the sort of variation that LaTeX2e is supposed to prevent happening, by defining a standard base installation and allowing only very restricted site configuration). Running LaTeX2e in compatibility mode will take this into account, and process files much as LaTeX run here would. It is possible to override this, to process old files from other sites if you need to, by using a file calledlatex209.cfg.
Please read texdoc latex and The LaTeX Companion for more details.

The new TeX system is part of your default environment, but to make it easy to switch between the different versions of the programs, there is a command:

      setup oldtex

which reverts back to the old programs, and restores the previous value ofTEXINPUTS. The setup mechanism doesn't handle any other environment variables --- if you have set BIBINPUTS, TEXFONTS or such-like, it would be best to unset them yourself, and study Guide to the Local TeX and LaTeX Installation (TN51) to see what to do.

For further information on LaTeX2e:

The latest edition of Lamport's book, LaTeX: A Document Preparation System.
The LaTeX Companion, by Goossens, Mittelbach and Samarin. The LaTeX Companion contains a description of advanced aspects of LaTeX, including how to customize document layout in various ways, and descriptions of many packages.
The Guide to the Local TeX and LaTeX Installation (TN51) here in the CS department.
The the local LaTeX FAQ a (currently small) list of Frequently Asked Questions.
Local LaTeX help.
The LaTeX2e for Authors (TN52).