LaTeX2e<#522#><#522#> can process (almost) any LaTeX~2.09 document, by entering
<#523#>LaTeX~2.09 compatibility mode<#523#>. Nothing has changed, you run
LaTeX<#524#><#524#> in the same way you always did, and you will get much the
same results.
The reason for the `almost' is that some LaTeX~2.09 packages made use
of low-level unsupported features of LaTeX. If you discover such a
package, you should find out if it has been updated to work with
LaTeX2e. Most packages will still work with LaTeX2e---the easiest way
to find out whether a package still works is to try it!
LaTeX~2.09 compatibility mode is a comprehensive emulation of
LaTeX~2.09, but at the cost of time. Documents can run up to 50
slower in compatibility mode than they did under LaTeX~2.09.
In addition, many of the new features of LaTeX2e<#525#><#525#> are not available in
LaTeX~2.09 compatibility mode.