Version 1.1 of / was designed to work with 2.09, which became obsolete as of June 1994; version 1.2 of / is designed to work with the new , aka LATEX2e. Generally speaking, pre-existing documents that were written for 2.09 can be processed by current through a `compatibility mode' that is automatically entered when a document uses documentstyle instead of documentclass (see below).
For generic the emulation provided by 's compatibility mode is extremely good—at least for well-formed documents that avoided commands not documented in the manual; the most common point of failure is due to use of internal font commands such as fivrm or tenrm, or to the loading of an option file that uses old internal commands (font-related or otherwise). For third-party extensions such as / the quality of the emulation in compatibility mode varies; at the time of writing (18-Jan-1995) the emulation is fairly good for AMS packages such as amstex or amssymb, but less good for the AMS documentstyles amsart and amsbook; work to improve the accuracy of the emulation is on-going.
In general, version 1.2 of / adheres as faithfully as possible to the conventions and command set described in the second edition of : A document preparation system (Lamport, 1994). For end users the changes lie primarily in the use of documentclass and usepackage instead of documentstyle plus options, and in the changed syntax of font commands—e.g., |...| and |...| instead of |...|.
For example, to use the amsbsy package with a general document class such as article, you should no longer write
\documentstyle[amsbsy]{article}% obsolete
but write instead
\documentclass{article}
\usepackage{amsbsy}
For pre-existing documents running in compatibility mode the old forms bf, it, etc., for the standard font commands now behave as described in the first edition of the manual. Depending on the installation options used for your copy of / version 1.1, this may be the `right' behavior for your existing / 1.1 documents; if not, you will need to update the options list of the documentstyle command by adding newlfont, for example:
\documentstyle[newlfont]{amsart}
Internally the changes in version 1.2 of / were somewhat more extensive; various parts were rewritten to take advantage of the most recent version of the font selection scheme, for example, as well as the new methods for managing packages and document classes, for defining robust commands, for writing technical commentary in typesettable form, and for providing installation scripts. In other words this version of / is thoroughly integrated with current , not just jury-rigged.
The user's guide for / 1.2 describes only the current behavior; it doesn't attempt to describe the ways in which current behavior differs from the behavior of previous versions (that's the purpose of this document).